Too much water for the blend
CommonGluten-free formulations need considerably more water than wheat ones, which makes over-shooting easy and its signature confusing. Excess free water keeps the starch gel soft, so the crumb reads as gummy and under-set even after a full bake, and the extra water has to leave during baking or it stays in the middle as a damp, close band.
The variable underneath
Hydration
How to tell after the fact: A tight, close crumb from a dough that was stiff throughout indicates hydration too low — most commonly when wholemeal or rye flour was used with a recipe written for white.
What to change
Use less liquid
Cut the liquid back, and check what the flours in your blend actually absorb — a starch-heavy blend takes considerably less water than a wholegrain one, so the same figure suits neither.
Applies where the crumb is damp and gummy and the bake reached temperature. It does not apply where the bake was simply short of time, which produces a similar-looking crumb for an entirely different reason.
Trade-off: Gluten-free bakes go dry quickly, and reducing water accelerates it. This is the fix most likely to trade one fault for another.