Adjust oven rack to middle position and preheat oven to 350°F. Spray two 9-by-5-inch loaf pans lightly with nonstick cooking spray.

In a large mixing bowl, whisk together flour, cornstarch, pumpkin pie spice, baking soda, salt, and xanthan gum. In separate bowl, mix together eggs, oil and orange juice. Add sugar and pumpkin. Blend until thoroughly combined.

Add pumpkin mixture to dry ingredients. Mix until ingredients are thoroughly combined and a smooth batter forms (use medium speed on a handheld mixer or medium-low speed on a stand mixer. If mixing by hand, use a balloon whisk and mix for two minutes). Scrape down sides and bottom of bowl once or twice with a rubber spatula while mixing.

Divide batter evenly between prepared loaf pans. Bake for 1 hour or until a toothpick inserted into the center of the loaf comes out clean.

Remove pans from oven and place on a wire rack. Allow bread to cool in the pan for 5 minutes. Remove bread from pans and place directly on wire rack. Allow bread to cool before slicing. Store bread wrapped in plastic wrap at room temperature for 3 to 5 days, or wrap and freeze for up to 3 months.