Line a large baking sheet with parchment paper or a silicone baking mat. In a medium bowl, add the sliced apples, brown sugar, gluten-free flour, lemon juice, cinnamon and nutmeg. Stir to evenly combine.
If the pie dough has been in the refrigerator longer than 1 hour, let it sit at room temperature for 15 minutes before rolling.
Place the dough on a pastry mat or a silicone baking mat. Roll the pie crust into a circle 12 inches in diameter. You may need to press it together if it starts to slit or break on the edges. This is normal, gluten-free pie crust requires a bit of patience to roll out.
After rolling, use the pastry mat to flip the dough onto the lined baking sheet.
Pour the apples and any juices in the center of the dough, leaving a 2–3-inch border. You can spoon them on or arrange the slices in a circular pattern. Fold the edges of the dough over the apples, overlapping where needed, and press gently to seal.
Refrigerate the shaped galette for 15-20 minutes while the oven preheats. Place a pizza steel in the oven. (A pizza steel helps to ensure a crispy bottom crust.) Preheat the oven to 400°F.
In a small bowl, whisk together the egg, water and pinch of salt for the egg wash.
Using a pastry brush, lightly brush the edges of the galette with egg wash. Sprinkle with sanding sugar, if desired.
Bake for about 35-40 minutes, until the pie crust is golden and the apples are bubbling.
Remove from the oven and let cool on the baking sheet for 15 minutes before serving. Drizzle with salted caramel sauce before serving if desired.