Adjust oven rack to middle position and preheat oven to 325°F. Whisk together granulated sugar, white rice flour, potato starch, tapioca starch, baking powder, salt, and xanthan gum in medium bowl. Set aside. In large mixing bowl, whisk together egg yolks, water, vegetable oil, and vanilla extract. Add whisked dry ingredients. Mix until thick batter is thick and smooth.

In bowl of stand mixer fitted with whisk attachment, combine egg whites and cream of tartar. Whip on high speed until medium peaks form. Add whipped egg whites, one quarter at a time, to batter. Fold, taking care not to deflate the batter, until no large lumps of egg whites remain.

Spoon batter into ungreased 10-inch tube (angel food) pan. Bake until cake is golden brown and springs back to the touch, about one hour. Remove from oven and immediately invert pan either onto legs that are on the pan or onto the neck of a beer or wine bottle. Allow cake to cool in the pan upside down.

To remove cake from pan, run a knife around the outside of the cake and around the tube. Turn cake onto a serving plate. Store cake, covered, on the counter for up to four days.