Preheat your oven to 350°F (175°C). Grease a 9x5 inch loaf pan and line it with parchment paper.
In a medium bowl, whisk together the gluten free baking flour, almond flour, baking powder, baking soda, salt, and cinnamon.
In a large bowl, whisk the melted butter and dark brown sugar together until smooth and slightly thickened. Add the eggs one at a time, whisking after each. Stir in the vanilla extract, maple syrup, and sour cream until well combined.
Fold the dry ingredients into the wet ingredients using a spatula until just combined. The batter should be thick and smell like brown sugar cookie dough with a hint of maple.
Pour the batter into the prepared pan and smooth the top. Sprinkle the turbinado sugar evenly over the surface for a crackly, cookie-like crust.
Bake for 50 to 60 minutes, until the top is deep golden and a toothpick inserted in the center comes out with just a few moist crumbs. Let the bread cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.
Once the bread has cooled, whisk together the powdered sugar, maple syrup, and milk until smooth and thick but pourable. Drizzle the glaze over the top of the bread in a back-and-forth motion. Let the glaze set for about 10 minutes before slicing.