Preheat oven to 325 degrees F (165 degrees C). Grease a large loaf pan.
Combine zucchini, eggs, oil, and vanilla extract in a blender; pulse until mixture resembles a milkshake.
Whisk together white sugar, white rice flour, sweet rice flour, cornstarch, tapioca, baking powder, cinnamon, baking soda, xanthan gum, and salt in a large bowl. Stir zucchini mixture into flour mixture until batter is well blended; pour into prepared loaf pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 1 hour. Cool in the pan for a few minutes before removing to cool completely on a wire rack.
Mix confectioners' sugar and lemon juice in a small bowl to form a thin glaze.
