Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper.
In a medium bowl, whisk together the gluten-free flour blend, baking soda, baking powder, salt, and cinnamon. Set aside.
In a large bowl, use an electric mixer to cream the softened butter and granulated sugar until light and fluffy, about 2-3 minutes. Add the eggs one at a time, beating well after each addition.
Mix in the mashed bananas, Greek yogurt (or sour cream), and vanilla extract until fully combined. The mixture may look slightly curdled, which is normal with gluten-free batters.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Do not overmix. If using, fold in the chopped walnuts or chocolate chips with a spatula.
Pour the batter into the prepared loaf pan and smooth the top. Bake for 55-65 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. If the top browns too quickly, tent with foil during the last 15 minutes. Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely. Slice and enjoy your gluten-free banana bread!