Preheat oven to 350 degrees and spray a 9x5 inch loaf pan with non-stick spray.

In a large bowl, whisk together the flour, xanthan gum, baking powder, baking soda, salt, granulated sugar, and brown sugar. Break up any clumps of brown sugar if necessary. Stir in the chopped walnuts.

In a separate bowl, whisk together the oil, eggs, milk, mashed banana, and vanilla extract. Pour the wet ingredients into the dry ingredients and stir to combine.

Pour the batter into the prepared loaf pan and bake at 350 degrees for 50-60 minutes or until a toothpick or cake tester comes out clean. Cooking time will vary depending on your oven - mine is usually done around 55 minutes.

Cool the bread in the pan on a cooling rack for 15 minutes then remove from the pan and cool completely on the cooling rack. To store, wrap tightly in plastic wrap and store in a zip-lock bag. It will stay fresh up to 3 days. This bread also freezes well.