I'm pretty new to gluten free baking, but according to some research I've been doing the cause of this problem is the different starch profiles found in gluten free grains. They absorb water at different rates and in different amounts than wheat flour.
So if we have a conventional recipe, we have to change the quantity of water to compensate for the change in flour. We also have to change the amount of time we give the dough or batter to absorb the water and rehydrate the starches in the GF flour.
I'm still learning about the changes to the amounts of water in recipes. But, the most successful GF recipes I have tried share a 30 minute rest period for any batter or dough before it's cooked.