Prep the pan. Heat the oven to 350°F (175°C) with a rack in the center. Grease a 9x5-inch loaf pan and line it with a strip of parchment that hangs over the long sides.
Whisk the dry ingredients. In a medium bowl, whisk the gluten-free flour blend, almond flour, baking powder, baking soda, salt and cinnamon until evenly mixed.
Mash the bananas. In a large bowl, mash the bananas with a fork until mostly smooth, with a few small lumps. Measure 1½ cups.
Add the wet ingredients. Whisk the melted butter, brown sugar, granulated sugar, eggs, yogurt and vanilla into the bananas until smooth and glossy.
Combine. Add the dry ingredients and stir with a spatula until no dry pockets remain. There's no gluten to toughen, so make sure the batter is completely smooth. Fold in the nuts or chips, if using.
Rest the batter. Scrape the batter into the pan, smooth the top and let it sit for 20 minutes at room temperature. It will thicken noticeably as the flours absorb liquid.
Bake. Bake for 65 to 75 minutes. Check at 40 minutes, and tent loosely with foil if the top is already deep brown. The loaf is done when an instant-read thermometer in the center reads 205°F (96°C) and a toothpick there comes out with a few moist crumbs but no wet batter.
Cool completely. Cool in the pan for 15 minutes, lift the loaf out by the parchment, and cool on a wire rack for at least 1½ hours before slicing.