Rinse the blueberries under cool water, pat dry, and mix with 1/2 cup sugar, lemon juice, and vanilla extract. Set aside.
In a separate bowl, combine gluten-free flour, baking powder, salt, and remaining sugar. Add melted butter and milk, mix until just combined.
Grease a 9x9-inch baking dish, pour in the batter, and spoon the blueberry mixture on top. Sprinkle with cinnamon.
Preheat oven to 350°F. Bake for 35-40 minutes until golden brown and a toothpick comes out clean. Cool for 10 minutes before serving.