Preheat the oven to 325 degrees F (160 degrees C). Line a 9x5-inch bread pan with parchment paper, so that the paper sticks up the two long sides of the pan.
In a large mixing bowl, whisk together the rice flour, millet flour, tapioca starch, cinnamon, baking soda, xanthan gum, and salt.
In a separate bowl, whisk together the sugar, oil, brown sugar, eggs, milk, and vanilla extract.
Pour the sugar mixture over the dry ingredients, and stir to combine.
Stir in the shredded zucchini and nuts (if using). Scrape batter into the prepared baking pan, spreading to make the loaf even.
Bake in the preheated oven for 60-65 minutes, or until a toothpick inserted into the middle of the loaf comes out clean.
Wait 5 minutes before removing the loaf from the baking pan, and placing it on a wire cooling rack. Once the loaf is completely cool, wrap in two layers of plastic wrap. It is good on the counter for two days, but since it is such a moist loaf of bread, it is better to store in the refrigerator.
When using shredded zucchini, do NOT squeeze the water out of the zucchini before using it. This will remove the liquid that you are relying on to give you a moist loaf.
Recipe by The Baking Beauties at http://www.thebakingbeauties.com/2014/06/gluten-free-zucchini-bread-2.html