Preheat the oven to 350°F and grease a 9x13 inch pan. Set aside.
To make the filling, in a large bowl, stir the peach chunks, brown sugar, cornstarch, lemon juice, vanilla, cinnamon, nutmeg, and salt together until combined. Pour filling into prepared baking pan and bake for 10 minutes. Remove from the oven and set aside to prepare the topping.
Whisk the gluten free flour, sugar, baking powder, baking soda, and salt together in a large bowl. Add the butter. Using a pastry cutteror two forks, cut the butter into the mixture until its similar to a coarse meal. Slowly drizzle in the buttermilk, stirring until combined. Gently flatten out handfuls of the dough and place on top of the peach feeling in a somewhat even layer. Some peach filling showing is okay.
Brush the top of the biscuit dough with egg wash, then sprinkle with cinnamon-sugar topping.
Bake for 43-47 minutes, or until lightly browned and biscuit topping is cooked through. Allow to cool in the pan on a wire rack for 5-10 minutes before serving. Serve warm with vanilla ice cream or whipped cream.
Baked cobbler can be stored in the freezer for up to 3 months. Remove and allow to set at room temperature for 1 hour then bake at 350°F for 30-35 minutes.