Make pizza crust
In bowl of electric mixer, whisk together tapioca flour, white rice flour, chickpea flour, sorghum flour, xanthum gum, and salt.
In small saucepan over moderate heat, stir together milk and 1/4 cup water and heat until warm but not hot to the touch, about 1 minute (the mixture should register between 105°F and 115° F on candy thermometer). Stir in yeast and sugar. Add milk–yeast mixture, egg whites, and 2 tablespoons oil to dry ingredients and, using paddle attachment, beat at medium speed, scraping bowl occasionally, until dough is very smooth and very thick, about 5 minutes.
Remove racks from oven, set pizza stone or heavy upturned baking sheet on bottom of oven, and preheat to 400°F. (Preheat at least 45 minutes if using pizza stone or 20 minutes if using baking sheet.)
Have ready two 12-inch squares parchment paper. Scrape half of dough onto each square and form each half into a ball. Coat each ball with 2 teaspoons oil, then use oiled fingertips to pat and stretch each ball into 9-inch-diameter round, 1/4 inch thick, with a 1/2-inch-thick border. Loosely cover rounds with plastic wrap and let rise in warm draft-free place until each pizza is about 10 inches in diameter, about 20 minutes.
Using baking peel, transfer 1 crust with parchment to preheated pizza stone and bake until top is puffed and firm and underside is crisp, 5 to 10 minutes. Using baking peel and discarding parchment paper, transfer baked crust to rack to cool. Bake second crust in same manner. (Baked crusts can be made ahead and frozen, wrapped in plastic wrap, up to 1 month. Thaw in 350°F oven until hot, 4 to 5 minutes, before topping and broiling.)