Preheat the oven to 350F. Grease two 8-inch round pans or one 9x13 pan. I also like to line the bottom of the 8-inch round pans with a circle of parchment paper then grease again.
In a medium bowl, whisk together the gluten-free flour, baking powder and salt.
In the bowl of a stand mixer, add the eggs and sugar. 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.) Mix in the vanilla and lemon juice.
With the mixer on low, add ⅓ of the dry ingredients. Mix until incorporated. Add the sour cream and mix to combine. Add another ⅓ of the dry ingredients and continue to mix. Add ½ of the milk followed by the last ⅓ of the dry ingredients. Add the remaining milk and stir until totally combined. (You want to add the dry ingredients alternately with liquid ingredients so the flour absorbs into the batter.)
Pour the batter into the prepared pans. Bake for 35-40 minutes until golden and a toothpick inserted into the center of each cake comes out clean.
If using 8-inch round pans, let the cakes cool in the pans for 15 minutes. Then turn out the cakes onto a wire rack to continue cooling. Cool completely before frosting.