Prepare the pie crust by placing a store-bought gluten-free pie crust in a pie dish or making your own with gluten-free flour. Poke holes in the bottom and optionally pre-bake for 10 minutes at 375°F.
In a large bowl, combine sliced apples with lemon juice, granulated sugar, ground cinnamon, ground nutmeg, salt, and cornstarch. Toss until apples are well coated and pour into the prepared pie crust.
In a separate bowl, mix rolled oats, brown sugar, gluten-free flour, melted butter, and vanilla extract until crumbly. Sprinkle over the apple filling.
Bake the pie in a preheated oven at 375°F for 45-50 minutes until apples are tender and topping is golden brown. Let cool for at least 15 minutes before slicing.