Preheat oven to 375. Line a regular sized muffin tin with 18 paper muffin liners. Grate the zucchini on the largest hole of your grater. Blot the zucchini with a paper towel or kitchen towel lightly to remove excess moisture. Set aside.
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.
Add the wet ingredients into the dry ingredients. Stir until no more streaks of flour remain.
Add in the grated zucchini. Mix until combined.
Pour the batter into the prepared muffin cups, leaving 1/4-inch from the top. Sprinkle the top of the muffin batter with turbinado sugar.
Bake at 375F on the center rack for 18-22 minutes, until the zucchini muffins are cooked through and a toothpick inserted into the center comes out clean with just a few crumbs.
Set the muffins to cool on a wire rack completely before slicing.