Preheat oven to 325 degrees F. Coat one 8" x 4" nonstick loaf pan with cooking spray. Set aside.
Using an electric mixer and a large mixing bowl, cream together the butter and sugar on medium speed until light and fluffy. Mix in the eggs on low speed until blended. Add the grated lemon peel.
In a medium bowl, sift together the flours, xanthan gum, baking powder, baking soda, and
salt. In another medium bowl combine the buttermilk and vanilla. On low speed, beat the dry ingredients into the butter mixture, alternating with the buttermilk -- beginning and ending with the dry ingredients. Mix just until combined. Spoon the batter into the prepared pan and smooth top.
Bake the cake for about 50 minutes or until top in golden brown and a cake tester inserted into center comes out clean. Cool cake in pan for 5 minutes, then remove from pan and cool on rack.