Prepare the Topping: In a small bowl, combine the granulated sugar and ground cinnamon for rolling. Set aside.
Make the Doughnuts: Whisk together white rice flour, potato starch, tapioca starch, granulated sugar, baking soda, baking powder, salt, xanthan gum, and nutmeg.
Add egg and buttermilk. Mix until a dough forms. (Use medium-high speed on a handheld mixer and medium speed on a stand mixer.) Dough will be thick and sticky.
Repeat. And be sure to use the doughnut dough scrapes for doughnut holes! Don't let it go to waste.