Preheat & Prep. To start, preheat oven temperature to 350 degrees F and arrange a rack in the center of the oven. Make or set out your pie crust in a 9-inch pie pan (I recommend a metal pie pan). Set out a baking sheet for later to catch any possible drips or spills.
Combine Filling. In a large bowl, by combining apples, gluten-free flour, brown sugar, cinnamon, vanilla, and salt. Stir to combine, until the apples are evenly coated. Pour apple pie filling into pie shell and gently shake a bit to let the apples settle. I like to make sure the apple slices are fairly closely packed so the filling doesn't collapse as much as the pie cools.
Next, Make The Crumble Topping. In a medium bowl (or the same bowl you used for the filling!), combine brown sugar, butter, oats, flour, vanilla, and salt. Mix with a pastry blender, two forks, or clean hands until the dry ingredients are evenly coated in butter and small clusters form.
Layer. Carefully scoop or spoon the crumble topping on top of the apples, and gently pack it together slightly with your hands. (Just a tiny bit of gentle pressure to help it stick together!) The pie will be very tall at this point!
Bake. Place the pie pan on the sheet pan to catch any spills or drips in the oven and bake for 45-55 minutes, or until the pie filling is bubbling around the edges and the top is golden brown. Check your pie at the 30 minute mark and again at the 45 minute mark to check for browning. If your crust is browning too quickly, you can carefully tent the crust with strips of aluminum foil, or move the pie to a lower rack in the oven.
Cool. Allow the pie to cool completely before cutting and serving. (If you slice into a hot or warm pie, the filling may not be properly set yet!)
Serve & Store. Cut into slices and serve individual slices with a scoop of vanilla ice cream and drizzle of caramel sauce, as desired. Cover the leftover pie and refrigerate 2-3 days.