Preheat the oven to 350°F and grease a 9×13 inch baking pan.
In a medium bowl, whisk together the gluten-free flour, baking powder, baking soda, cinnamon and salt.
In a separate medium bowl, mash the bananas. Add the sour cream, milk, vanilla and lemon juice to the bananas and mix until combined.
In the bowl of a stand mixer (or using a large bowl and hand mixer), add the white sugar, brown sugar and eggs. Whip on medium-high speed for 3 minutes, until light and fluffy. With the mixer running on low, slowly drizzle in the oil. (You want to do this slowly so you don't loose the air that you've mixed in.)
With the mixer on low, add ½ of the dry ingredients. Mix until incorporated. Add ½ the banana mixture and mix until combined. Add the remaining ½ of the dry ingredients and continue to mix. Mix in the rest of the banana mixture. Mix the batter until well combined, then scrape down the sides and bottom of the bowl and mix again. (You want to add the dry ingredients alternately with liquid ingredients so the flour absorbs into the batter.)
Pour the batter into the prepared pan. Bake for 35-40 minutes until golden and a toothpick inserted into the center of the cake comes out clean.
Remove the cake from the oven and set on a wire rack. Allow to cool completely. After about 45 minutes, I usually place it in the refrigerator to speed things up.