Preheat oven to 350F. Grease and line a 9x5-inch loaf pan.
In a medium bowl, whisk together the gluten-free flour blend, xanthan gum (if using), baking powder, baking soda, salt, ground cinnamon, and nutmeg. Set aside.
In a separate bowl, add the granulated sugar, brown sugar, vegetable oil, unsweetened applesauce, eggs, and vanilla. Whisk until combined.
Blot the zucchini with a paper towel or kitchen towel lightly to remove excess moisture. You do not need to wring it out completely. Note: The 2 cups is measured before it is blotted. After blotting, the zucchini will weigh approximately 200-220g. Whisk the zucchini into the wet ingredients.
Add the wet ingredients into the dry ingredients. Stir until no more streaks of flour remain.
Pour the batter into the prepared loaf pan. Sprinkle with turbinado sugar.
Bake at 350F on the center rack for 55-70 minutes, until the zucchini bread is cooked through and a toothpick inserted into the center comes out clean with just a few crumbs. The internal temperature of the bread should be 200°F (93°C) when taken with a thermometer. Cover the bread with aluminum foil if it is browning too quickly.
Set the bread to cool on a wire rack completely before slicing.