Preheat oven to 350 degrees F (175 degrees C). Grease a 9x5-inch loaf pan.
Sift flour, sugar, baking powder, xanthan gum, cinnamon, salt, and baking soda together in a large mixing bowl. Beat pumpkin, eggs, and butter together in a separate bowl; add to flour mixture and beat until the mixture blends into a batter. Stir walnuts through the batter.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 1 hour.