Our verdict

The Brooks Glycerin GTS 21 is a robust and well-constructed daily trainer, ideal for those requiring moderate stability. It's extremely cushioned, comfortable, and supportive, with an outsole that lasts forever. However, we also noticed that the upper tends to run warm and the shoe doesn’t perform well at faster speeds, limiting its versatility across different weather conditions or paces. Despite these issues, we think it's still an exceptional pick for those seeking some support without committing to a full stability model.

Pros

  • Reasonable weight gain over non-GTS version
  • Super comfortable upper
  • Fantastic non-intrusive stability
  • Responsive supercritical foam
  • Built-like-a-tank outsole
  • Roomy toebox
  • Ready for marathon training

Cons

  • Knit upper lacks ventilation
  • Not suitable for faster paces
  • Could be lighter

Audience verdict

86
Great!
  • Top 29% most popular running shoes

Who should buy

We've discovered in the lab that the Brooks Glycerin GTS 21 is a superb choice for:

  • Runners who desire a daily trainer that provides fantastic stability without the clunkiness or extra weight typically associated with such features.
  • Neutral runners in need of enhanced support, particularly useful during the final miles of long runs.
  • Mild-pronators or those with recurrent shin splints issues looking for a shoe with ample cushioning and extraordinary durability.

Brooks Glycerin GTS 21

Who should NOT buy

Brooks has done a fantastic job with this shoe, but we've found in the lab that it has certain drawbacks, particularly its weight. For instance, shoes like the Hoka Arahi 7 and the Brooks Adrenaline GTS 23, which we also tested here in the lab, are be better options if you're not looking for a max-stack shoe.

Additionally, we do not recommend this model for most runners with a forefoot striking technique due to its high drop. We suggest the Saucony Tempus as an alternative with less heel-to-toe drop, which we think could better suit that running style.

Brooks Glycerin GTS 21 parts

Breathability

The heel counter earned a perfect score of 5 out of 5 in our manual evaluation—a result that should hardly be surprising for a stability shoe.

Brooks has enhanced the GTS by incorporating an additional TPU piece on the outer heel, boosting rigidity and providing crucial stability for rearfoot strikers.

Test results
Glycerin GTS 21 3
Average 3.8
Compared to 227 running shoes
Number of shoes
1
Breathability
5

Durability

Toebox durability

Our initial test using the Dremel on the GTS 21 focused on assessing the toebox's durability. We awarded it a modest score of 3 out of 5, which is a positive result, especially for a knit upper.

We're quite satisfied with this, considering the Glycerin's luxuriously plush upper. This performance confirms our belief that the shoe offers decent resilience, especially given the softness of its materials.

Brooks Glycerin GTS 21 Toebox durability
Test results
Glycerin GTS 21 3
Average 2.4
Compared to 161 running shoes
Number of shoes
1
Toebox durability
5

Heel padding durability

After completing the evaluation of the toebox, we shifted our focus to the heel padding to see if the results would be as promising.

We were quite pleased with what we discovered—a score of 4 out of 5. This excellent rating reflects outstanding durability in the heel, underscoring the GTS robust Achilles area.

Brooks Glycerin GTS 21 Heel padding durability
Test results
Glycerin GTS 21 4
Average 3.1
Compared to 157 running shoes
Number of shoes
1
Heel padding durability
5

Outsole hardness

We turned the shoe over to examine the outsole closely, starting with the hardness of the Roadtack Rubber. The GTS variant, designed specifically for mild-stability runners, includes additional coverage in the midfoot to mitigate wear from overpronation.

Brooks Glycerin GTS 21 rubber

Our tests showed the Roadtack Rubber has a hardness of 87.0 HC, emphasising that this outsole is primarily built for lasting durability over everything else.

Brooks Glycerin GTS 21 Outsole hardness
Test results
Glycerin GTS 21 87.0 HC
Average 80.5 HC
We use an average of four tests. The photo shows one of those tests.
Compared to 278 running shoes
Number of shoes
54.9 HC
Outsole hardness
92.8 HC

Outsole durability

In our final Dremel test, we doubled the speed to 10K RPM to challenge the Roadtack Rubber outsole of the Glycerin GTS 21 more aggressively.

Remarkably, the rubber of the Glycerin GTS 21 has proven itself to be exceptionally resilient. Despite the intensified testing, we noted only a slight indentation of 0.5 mm. That's an A++ in our books.

Brooks Glycerin GTS 21 Outsole durability
Test results
Glycerin GTS 21 0.5 mm
Average 0.9 mm
Compared to 139 running shoes
Number of shoes
0.0 mm
Outsole wear
2.0 mm

Outsole thickness

We measured the rubber at a substantial thickness of 3.2 mm, which seems a lot, especially considering the minor 0.5 mm indentation we observed just before.

Brooks Glycerin GTS 21 Outsole thickness
Test results
Glycerin GTS 21 3.2 mm
Average 3.2 mm
Compared to 297 running shoes
Number of shoes
0.0 mm
Outsole thickness
6.6 mm

Weight

Despite weighing 10.6 oz (301g), we were quite pleased with the outcome, as it's only 0.8 ounces heavier than the standard version.

Given the additional rubber in the heel, the GuideRails system, and the reinforced heel counter, the slight increase in weight is quite reasonable!

Brooks Glycerin GTS 21 Weight
Test results
Glycerin GTS 21 10.62 oz (301g)
Average 9.38 oz (266g)
Compared to 298 running shoes
Number of shoes
5.26 oz (149g)
Weight
12.56 oz (356g)

Cushioning

Heel stack

In the heel, we measured 37.2mm following World Athletics guidelines, firmly positioning the Glycerin as a max-stack, mild-stability running shoe.

While this might initially spark concerns about getting proper support, we can confidently say it does a great job—it's impressively cushioned and remarkably stable.

Brooks Glycerin GTS 21 Heel stack
Test results
Glycerin GTS 21 37.2 mm
Average 33.6 mm
Compared to 297 running shoes
Number of shoes
7.6 mm
Heel stack
42.7 mm

Forefoot stack

Our measurements showed the forefoot thickness at 26.5 mm.

Brooks Glycerin GTS 21 DNA Loft

Although this dimension offers a closer feel to the ground compared to the thicker heel, it still delivers generous cushioning for forefoot and midfoot strikers.

Brooks Glycerin GTS 21 Forefoot stack
Test results
Glycerin GTS 21 26.5 mm
Average 25.0 mm
Compared to 297 running shoes
Number of shoes
7.6 mm
Forefoot stack
35.8 mm

Drop

The gap between the two prior measurements results in an actual 10.7-mm drop, which nearly matches the official 10-mm drop claimed by Brooks.

By the way, if you're curious about why there are often discrepancies between our findings and the brands' claims, we offer a detailed guide that fully explores this topic.

Brooks Glycerin GTS 21 Drop
Test results
Glycerin GTS 21 10.7 mm
Average 8.6 mm
Compared to 297 running shoes
Number of shoes
-0.8 mm
Drop
16.1 mm

Insole thickness

We found a fairly typical EVA insole with a thickness of 4.8 mm. It's delivers essential cushioning without distinguishing itself by being ultra slim or thick.

Brooks Glycerin GTS 21 Insole thickness
Test results
Glycerin GTS 21 4.6 mm
Average 4.5 mm
Compared to 293 running shoes
Number of shoes
1.5 mm
Insole thickness
7.3 mm

Midsole softness

Note: a low durometer measurement equals a soft material, whereas a high measurement means it's firm.

One aspect Brooks could have considered tweaking in this GTS version is the foam. However, we found that it remains unchanged, featuring the same foam that is used throughout the Glycerin series. From our perspective, opting for a slightly firmer compound could have enhanced stability even more for this model.

Consequently, we discovered that the GTS version continues to use the nitrogen-infused EVA known as DNA Loft v3 (20.9 HA). It's a reliable foam that offers sufficient energy return and pop for everyday running.

Brooks Glycerin GTS 21 Midsole softness
Test results
Glycerin GTS 21 20.9 HA
Average 21.5 HA
We use an average of four tests. The photo shows one of those tests.
Compared to 225 running shoes
Number of shoes
8.5 HA
Midsole softness (soft to firm)
41.5 HA

Difference in midsole softness in cold

To our dismay, after putting the shoe in the freezer for a 20-minute nap, the softness had changed by 37.1%—a noticeable and marked increase for a £170 shoe. This test highlighted a significant hardness in the foam when exposed to cold temperatures, which could affect performance for those who live in colder climates.

Brooks Glycerin GTS 21 Difference in midsole softness in cold
Test results
Glycerin GTS 21 37.1%
Average 25.6%
Compared to 224 running shoes
Number of shoes
0%
Difference in midsole softness in cold
100%

Rocker

The Glycerin saga has consistently delivered a natural-ride, barely-rockered running experience, and we've observed that it continues this tradition. We consider this a triumph, as it preserves the shoe's core DNA.

Brooks Glycerin GTS 21 profile

With the market overflowing with rockered shoes, offering a variety of options like this is essential for runners. And it's the right choice for a mild-stability shoe.

Stability

Lateral stability test

The "GTS" in the shoe's name stands for "Go To Support," and indeed, support is where this shoe truly excels.

Brooks Glycerin GTS 21 Go To Support

It's important to note that Brooks doesn't market this as a stability shoe for severe overpronators, but rather as a hybrid—somewhere between a robust stability shoe and a daily trainer.

It's exceptionally effective for runners with moderate stability needs, subtly enhancing gait correction and promoting a smoother stride without being obtrusive.

Torsional rigidity

One standout feature when comparing the GTS with the regular edition is its enhanced torsional rigidity, crucial for its mild-stability role.

We scored this feature a robust 4 out of 5, which is significantly higher compared to the standard model's 2 out of 5. This difference highlights the GTS version's focus on providing more support over a flexible ride.

Test results
Glycerin GTS 21 4
Average 3.2
Compared to 276 running shoes
Number of shoes
1
Torsional rigidity
5

Heel counter stiffness

The heel counter earned a perfect score of 5 out of 5 in our manual evaluation—a result that should hardly be surprising for a stability shoe.

Brooks has enhanced the GTS by incorporating an additional TPU piece on the outer heel, boosting rigidity and providing crucial stability for rearfoot strikers.

Test results
Glycerin GTS 21 5
Average 2.8
Compared to 260 running shoes
Number of shoes
1
Heel counter stiffness
5

Midsole width in the forefoot

Although the GTS version introduces some changes in rigidity and the outsole, the midsole dimensions are consistent with the neutral version.

We measured 117.3 mm in the widest part of the forefoot, providing stable support that benefits forefoot and midfoot strikers, especially during sharp turns.

Brooks Glycerin GTS 21 Midsole width in the forefoot
Test results
Glycerin GTS 21 117.3 mm
Average 113.7 mm
Compared to 298 running shoes
Number of shoes
100.5 mm
Midsole width in the forefoot
126.5 mm

Midsole width in the heel

The heel reveals a similar story—we encountered the same large silhouette found in the neutral daily trainer, measuring a significant 100.2 mm.

This dimension fits well within the expectations for a mild-stability running shoe, although it's notably large for a neutral trainer. It appears to us that Brooks opted to standardise the sole across both models, but using the GTS version as a foundational blueprint.

Brooks Glycerin GTS 21 Midsole width in the heel
Test results
Glycerin GTS 21 100.2 mm
Average 90.5 mm
Compared to 298 running shoes
Number of shoes
74.9 mm
Midsole width in the heel
106.6 mm

Flexibility

Stiffness

Initially, we were concerned about the GTS's torsional rigidity, but our worries were alleviated after conducting our 90-degree bend test.

The force gauge registered only 23.4N—a relatively low measurement that reassures us of its inherent flexibility. This confirms that the GTS is still suitable as a comfortable daily trainer, ideal for easy paces or even casual walking with your dog.

Test results
Glycerin GTS 21 23.4N
Average 29.1N
We use an average of four tests. The video shows one of those tests.
Compared to 280 running shoes
Number of shoes
2.2N
Stiffness
94.4N

Difference in stiffness in cold

After we left the Glycerin GTS 21 in the freezer for an additional 20 minutes and repeated our previous test, the results were very encouraging. We discovered that the shoe only became 15% stiffer—a surprisingly good performance. This is excellent for runners who face colder conditions.

Test results
Glycerin GTS 21 15%
Average 36.2%
Compared to 280 running shoes
Number of shoes
0%
Difference in stiffness in cold
148%

Size and fit

Toebox width at the widest part

We discovered the toebox to be really spacious at 99.7 mm, comfortably accommodating even slightly wider feet.

Brooks Glycerin GTS 21 feet

Additionally, Brooks offers the Glycerin GTS 21 in wide (2E) sizes in most countries, ensuring that those with broader feet can also enjoy the mild stability this version provides.

Brooks Glycerin GTS 21 Toebox width at the widest part
Test results
Glycerin GTS 21 99.7 mm
Average 98.4 mm
Compared to 298 running shoes
Number of shoes
89.5 mm
Toebox width at the widest part
109.1 mm

Toebox width at the big toe

You might have caught a glimpse of the big toe area in our previous test with our POV picture, but we went ahead and measured it. At 80.6 mm, we're thrilled to confirm that it's more spacious in the big toe area than many other daily trainers on the market, although don't expect it to be fit an Altra!

Brooks Glycerin GTS 21 Toebox width at the big toe
Test results
Glycerin GTS 21 80.6 mm
Average 78.2 mm
Compared to 172 running shoes
Number of shoes
60.4 mm
Toebox width at the big toe
92.5 mm

Tongue: gusset type

Unfortunately, our tests revealed that the Glycerin features a non-gusseted tongue, which we think is a notable miss.

We are convinced that incorporating a semi-gusseted tongue would significantly improve both the ride and feel of the shoe, especially considering the small price hike from its predecessor.

Brooks Glycerin GTS 21 Tongue: gusset type
Test results
Glycerin GTS 21 None

Comfort

Tongue padding

The tongue of the Glycerin GTS 21 is well-cushioned, featuring a plush, pillow-like main foam surrounded by a slimmer layer.

Brooks Glycerin GTS 21 laces

The lacing system is straightforward and functional, exactly what you'd expect from a daily trainer, free from any unnecessary additions or complicated designs.

We measured the padding at an impressive 11.5 mm! While we think this might be a bit excessive for some and could be used to shed some weight, we can confirm that it offers exceptional comfort.

Brooks Glycerin GTS 21 Tongue padding
Test results
Glycerin GTS 21 11.5 mm
Average 5.6 mm
Compared to 295 running shoes
Number of shoes
0.5 mm
Tongue padding
14.2 mm

Heel tab

The Glycerin GTS 21 does not feature a heel tab, consistent with most other Brooks running shoes.

Brooks Glycerin GTS 21 Heel tab
Test results
Glycerin GTS 21 None

Removable insole

We found that the insole is not fixed to the last, which makes it simple to exchange it with a third-party footbed.

Brooks Glycerin GTS 21 Removable insole
Test results
Glycerin GTS 21 Yes

Misc

Reflective elements

It came as a letdown to find out that the Glycerin 21 GTS lacks any reflective elements, much like its non-GTS counterpart. We are convinced that incorporating reflective details could significantly benefit runners who prefer late-evening or early-morning runs.

Brooks Glycerin GTS 21 Reflective elements
Test results
Glycerin GTS 21 No