Whisk together the flour, sugar, baking powder, salt, cream of tartar and xanthan gum. Cut in the shortening with a pastry cutter or fork.
Whisk together the egg and milk. Make well in center of dry ingredients. Pour egg mixture into well. Stir together just enough to moisten dry ingredients.
Pat out dough onto floured surface to one half inch thick. Cut biscuits and bake on ungreased cookie sheet at 450°F until lightly browned. Makes 18 biscuits.