Preheat your oven to 350°F. Grease an 8-inch or 9-inch baking dish with butter or non-stick cooking spray.
In a large bowl, gently toss the sliced peaches with cornstarch, sugar, lemon juice, vanilla extract, cinnamon, and salt until well combined.
Transfer the peach mixture to the prepared baking dish and spread it evenly.
In another bowl, combine the light brown sugar, gluten-free flour, cinnamon, and salt. Cut in the cold butter using a pastry cutter or two forks until the mixture resembles coarse crumbs.
Stir in the old-fashioned oats.
Sprinkle the topping evenly over the peach filling, covering it completely.
Bake for 40-45 minutes, or until the topping is golden brown and the filling is bubbling around the edges.
Remove the crisp from the oven and let it cool slightly before serving. Serve warm, topped with vanilla ice cream or whipped cream, if desired.
Notes
Different gluten-free flours may yield different baking results. Whenever I need a good all-purpose gluten-free flour, I typically use Pillsbury's gluten-free flour or Bob's Red Mill 1-to-1 Gluten-Free Baking Flour.
Use the "spoon & level" method to measure your gluten-free flour. Spoon the flour into your measuring cup and level it off with a knife. This prevents the flour from compacting in the cup and gives you a more accurate measurement.
Double-check that your oats are gluten-free. Not all oat brands are gluten-free.
To Store: Keep in an airtight container in the refrigerator for up to 3 days. Reheat in the microwave or oven at 350°F.
To Freeze: Let cool, and then portion into freezer-safe containers. Freeze for up to 3 months and thaw overnight in the fridge before reheating.