Preheat your oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment paper, allowing overhang on the sides for easy removal. Alternatively, grease the pan thoroughly with cooking spray or coconut oil.
In a medium bowl, whisk together the gluten-free flour blend, baking soda, baking powder, cinnamon, nutmeg, and salt for at least 30 seconds to ensure even distribution.
Peel and core the Granny Smith apples, then dice them into small, uniform ¼-inch cubes. For added brightness, toss the diced apples with 1 tablespoon of lemon juice if desired.
In a large bowl, whisk together the eggs, applesauce, melted coconut oil, granulated sugar, brown sugar, and vanilla extract until smooth and well incorporated.
Gradually add the dry ingredient mixture to the wet ingredients, stirring gently until just combined. Be careful not to overmix.
Fold in the diced apples, reserving a handful for the top if desired. The batter should be thick but pourable.
Transfer the batter to your prepared loaf pan. If using the cinnamon sugar topping, combine the sugar and cinnamon in a small bowl, then sprinkle evenly over the top of the batter. Add any reserved apple pieces on top.
Bake in the preheated oven for 55-60 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. If the top browns too quickly, tent with aluminum foil after about 35-40 minutes of baking.
Allow the bread to cool in the pan for 15 minutes before transferring to a wire rack to cool completely. This cooling period is crucial for gluten-free baked goods to properly set.