Syronai's mix should work well; you can mix and match gluten-free flours in a ratio of one-half medium weight flour, 1/4 part light flour and 1/4 cup starch such as tapioca; e.g. 1/2 cup brown rice flour, 1/4 cup millet flour and 1/4 cup tapioca starch with some guar gum or xantham gum (1/4 teaspoon) added.

If you're not so attached to making a GF biscuit for the cobbler, making a crisp/crumble topping is easier and can allow the GF flour flavor to really shine. If you really want to make a GF biscuit I suggest you don't roll them quite as thick as one would a wheat flour biscuit. {this is a few years old now, but it gives you an idea}

Wheat flour has more growing/rising/leavening properties and in order to "mimic" a GF approximation I fear you would have to add far too much baking powder than your mouth would desire as an end result.

Whole Foods carries a line of mixes called Pamela's. You could use the pancake mix. Make as for pancakes, pour over your fruit, sprinkle with a little sugar, and bake.

Most peach cobbler recipes call for 1 cup all-purpose flour. Sub for this: 1/2 cup brown rice, amaranth, or millet flour. 1/2 cup sweet rice flour. 2-3 Tbls tapioca or potato starch. It should work well. I'll see if I can find my own peach cobbler recipe when I'm home later today and I'll type it up for you.