Brooks Glycerin Flex vs Brooks Glycerin GTS 23:
Which to Choose

The single most important difference is support: the Glycerin Flex is a fully neutral trainer with a broad, flexible platform and no pronation guidance, while the GTS 23 adds Brooks' GuideRails and a wider, well-sidewalled base to gently rein in mild overpronation. That guidance comes with mass — the GTS 23 weighs about 306 g (men's US 9) versus roughly 264 g for the Flex, a real ~42 g gap you'll feel over a long run. The GTS 23 also sits taller and softer, on a 38 mm heel / 28 mm forefoot stack against the Flex's 35 mm / 26 mm, which is why it rates a notch higher on cushioning (5/6 vs 4/6) and feels a touch more protective at landing. Drop is effectively a wash at the foot — about 10 mm measured on the GTS 23 and 9 mm on the Flex (both lower than Brooks' marketed figures) — so both keep a traditional heel-forward geometry that eases the calves and Achilles. Neither is a speed shoe: both are plateless, both go vague if you force the pace, and both are happiest at conversational efforts over distances up to a 10K or half at relaxed effort. Where they diverge in feel is the transition — the Flex's deep flex grooves let the shoe bend naturally with your foot for a smooth, unstructured roll, whereas the GTS 23 delivers a slightly firmer, more supportive toe-off from its smaller front cells while the GuideRails keep everything planted.