Preheat oven to 350°F (175°C). Grease a 9-inch square or round baking pan.
In a medium bowl, whisk together the gluten-free flour blend, baking powder, baking soda, and salt.
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
Alternately add the dry ingredients and the sour cream (or yogurt) to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.
In a small bowl, mix the 2 tbsp of cinnamon with 1/4 cup of granulated sugar to create the swirl filling.
Pour half of the batter into the prepared pan. Sprinkle most of the cinnamon-sugar mixture over the batter.
Spread the remaining batter over the cinnamon-sugar layer.
For the crumb topping, combine the 1/2 cup gluten-free flour, 1/2 cup brown sugar, and 1/4 cup melted butter in a bowl. Mix until coarse clumps form. Sprinkle over the batter.
Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 15-20 minutes before transferring to a wire rack to cool completely.
