1. Preheat oven to 350°.
2. Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, xanthan gum, and next 4 ingredients (through salt) in a medium bowl; stir with a whisk.
3. Place banana and next 6 ingredients (through eggs) in a medium bowl; beat with a mixer at medium speed until blended. Gradually add flour mixture, beating at low speed until smooth.
4. Pour batter into a 9 x 5-inch loaf pan coated with cooking spray, and sprinkle with walnuts. Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Cool 15 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.