Preheat the oven to 375 degrees then spray a 9x13” baking pan with nonstick spray and set aside.
To a medium-sized mixing bowl add the berries, 1/2 cup granulated sugar, and a heaping Tablespoon gluten-free flour then fold gently with a spatula to combine and set aside.
To a large mixing bowl, add the remaining 3 cups gluten-free flour, remaining 1/2 cup granulated sugar, brown sugar, baking powder, cinnamon, and salt then stir with a fork to combine. Add the egg and vanilla extract to a small dish then whisk to combine. Drizzle the egg mixture over the flour mixture then stir with the fork to combine.
Add the cubed butter to the flour mixture then stir to combine. Switch to a pastry butter or use your fingertips to cut the butter into the flour mixture until only small crumbles of butter remain.
Scoop a little more then half the crumb mixture into the prepared baking pan then use your hands or the bottom of a measuring cup to press it into a firm layer. Pour the berries over the crust then spread them into an even layer. Squeeze handfuls of the remaining crumb mixture into clumps then crumble the clumps over the berries.
Bake the bars for 38-40 minutes or until the crumb topping is light golden brown, rotating the baking pan 180 degrees halfway through. Check on bars at the halfway - if the topping is browning too quickly, place a piece of foil on top. Uncover for the last 8-10 mins so the topping becomes golden brown.
Cool the bars completely then let them sit for at least 6 hours before cutting into small squares and serving. The bars will soften and become better as they sit. Store leftovers in the refrigerator.