Whisk together the flours, cornmeal, yeast, salt, and xanthan gum in a 5- quart bowl, or a lidded (not airtight) food container.
Combine the liquid ingredients with the sweetener and add the mixture to the dry ingredients, using a spoon, 14- cup food processor (with dough attachment), or a heavy-duty stand mixer (with paddle), until all of the dry ingredients are well incorporated. You may have to use wet hands to get the last bit of flour to incorporate if you’re not using a machine.
Cover (not airtight) and allow it to rest at room temperature until the dough rises, approximately 2 hours.
The dough can be used immediately after its initial rise, though it is easier to handle when cold. Refrigerate it in a lidded (not airtight) container and use over the next 5 days. Or store the dough for up to 2 weeks in the freezer in 1/2-pound portions. When using frozen dough, thaw it in the refrigerator overnight before use.
On pizza or flatbread day, break off a piece of dough, it will not have any stretchiness like wheat doughs. Roll out the dough directly on a pizza peel, covered generously with rice four. Sprinkle the surface of the dough with a little more rice flour and cover the dough plastic wrap. Roll into an 1/8-inch-thick round. As you are rolling out the dough, use a bench knife to make sure the dough isn’t sticking to the peel; add more flour if needed. Gently pull off the plastic wrap, and finish with your favorite toppings.