1. Preheat oven to 350°.
2. Weigh or lightly spoon flours and potato starch into dry measuring cups; level with a knife. Combine flours, potato starch, xanthan gum, and next 3 ingredients (through baking soda) in a medium bowl; stir with a whisk.
3. Place sugar and butter in a medium bowl; beat with a mixer at medium speed until well blended. Add eggs, 1 at a time, beating well after each addition. Beat in oil and vanilla. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture.
4. Spoon batter into an 8 x 4-inch loaf pan coated with cooking spray. Bake at 350° for 55 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pan on a wire rack.