Mash the overripe bananas until smooth. The riper the bananas, the more natural sweetness and flavor they will provide.
Whisk together the gluten-free flour blend, granulated sugar, brown sugar, and spices (cinnamon for the batter, nutmeg, or allspice if using).
Combine the mashed bananas, melted butter or oil, eggs, and vanilla extract. Mix well.
Gently fold the wet ingredients into the dry ingredients until just combined. Be careful not to overmix to prevent a tough texture.
Prepare the cinnamon swirl mixture by combining brown sugar and ground cinnamon in a small bowl.
Pour about half of the banana bread batter into a greased and floured loaf pan. Sprague half of the cinnamon-sugar mixture evenly over the batter.
Using a knife or skewer, gently swirl the cinnamon-sugar into the batter a few times (less is more to maintain distinct layers).
Pour the remaining batter over the swirled layer, then sprinkle the remaining cinnamon-sugar mixture on top. Swirl minimally again.
Bake in a preheated oven at 350°F (175°C) for approximately 50-60 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs but no wet batter. If the top browns too quickly, tent loosely with aluminum foil.
Let the banana bread cool completely on a wire rack before slicing and serving to ensure proper setting and moisture.
