Preheat oven to 350 F (180 C). Grease 1 large loaf pan.
In a large bowl, whisk together flours, brown sugar, baking powder, xanthan gum, baking soda, salt, and spices..
Carefully separate eggs, placing yolks in the dry ingredients bowl along with the pumpkin, melted butter, milk, and vanilla. Whisk until smooth and well combined.
Add walnuts, continue mixing just until everything is well combined. Set aside.
In a – very clean – mixing bowl or stand mixer, whip the egg whites on high speed, until stiff peaks form.
Gently fold whipped egg whites into the main batter, until all of the “white” disappears.
Pour batter into prepared loaf pan. Bake for about 60-70 minutes, or until crust is a rich golden brown, and knife inserted into middle of loaf comes out clean.
Best served warm.