Preheat the oven to 350°F.
In a large bowl, mix together the gluten free flour, brown sugar, granulated sugar, cinnamon and salt to combine.
Add in the cold, cubed unsalted butter and pinch the cubes with your hands until it is incorporated into the mixture. Continue to mix with your hands until the texture of the topping begins to resemble sand. Set it aside for a moment.
In another large bowl, add in the bite-sized apple pieces, cinnamon and sugar. Toss the apples in the cinnamon and sugar until completely coated.
Pour the apples into an 8" x 8" (20cm x 20cm) baking dish and distribute evenly.
Pour the crumble topping over the apples and ensure it covers them completely. There's no need to press the filling down, just allow it to coat the apples generously.
Bake the apple crumble in the oven for approximately 25-30 minutes until the apple filling is bubbling underneath.
Allow to cool slightly before eating.
