Preheat the oven to 170°C.
Peel the apples and chop them into smaller chunks. Pop them into a medium-sized, ovenproof dish.
In a separate bowl, toss together the remaining ingredients: gluten-free flour, oatmeal, sugar, vanilla extract, and cold butter, cut into small cubes.
Mix the ingredients together using your hands until the mixture becomes crumbly. Then, evenly scatter this over the apples you've prepared earlier. Place the dish in the oven and bake for about 30-40 minutes, or until the crumble is crisp and golden on top. If you find the crumble a bit pale towards the end, just crank up the heat to 200°C for the final few minutes.
Serve the crumble warm, perhaps with a scoop of vanilla ice cream. Enjoy!