Gluten is a combination of proteins that occur naturally in wheat
(and to a lesser extent, in rye and barley). When flour is
moistened, then kneaded or mixed, the gluten
molecules literally stretch out, forming longer and longer chains.
These chains are elastic, which is why you can stretch a piece of
dough. As the dough develops, the glutens form pockets that are
like stretchy balloons.
When the gases produced by yeast or another leavening agent
inflate these gluten balloons, the dough rises. And finally, baking
hardens the dough in its inflated state,
giving the bread its structure.
Now, with gluten-free flours such as rice flour, almond flour, or buckwheat flour, none of this is going to happen. With no gluten, the dough can't stretch.
The gases from the leavening seep out instead of causing it to rise, and the final product is generally significantly flatter than its gluten-based version.
This will obviously be less of a problem with foods like pancakes and cookies than it will with fluffy items like cakes and bread. Likewise, pizza dough relies on gluten for its characteristic chewiness and crunch.
Different flours can indeed be mixed to form a crust, but it will typically be thinner and more brittle.
As you get used to cooking with gluten-free flours, you'll get better at gauging what's possible and what's not. And there are a great many pre-packaged baking mixes
that take all the guesswork out of making these substitutions.
On the bright side, one of the major pitfalls with wheat flour, particularly with tender baked goods like pie crust, soft cakes, and quick bread, is that overworking the
dough or batter will cause the final product to be too tough. And with gluten-free flours, it's simply not possible to overwork the dough, crust, or batter, since there is no
gluten in it to overwork.