Preheat oven to 350˚.
Prepare gluten free pasta shells according to the package directions but cook 1 or 2 minutes less than the suggested cooking time.
Spray a 13x9 baking dish with non-stick cooking spray.
Spread 1-1½ cups pasta sauce in the bottom of the casserole dish.
In a large bowl, mix the ricotta, chicken, spinach, 1½ cups mozzarella, egg, basil, garlic powder, salt, and pepper until combined.
Fill each pasta shell with the mixture and place In the pan.
Pour the remaining sauce over the shells.
Cover pan with aluminum foil.
Bake for 30 minutes.
In a small bowl, melt 1 Tablespoon butter and combine with the breadcrumbs.
Uncover and sprinkle with Parmesan, bread crumbs, and remaining ½ cup mozzarella onto the shells.
Bake or broil until cheese is melted, about 5 minutes more.
Let stand for 10 minutes before serving.