Preheat your oven to 375°F (190°C).
Cook the gluten-free jumbo pasta shells according to the package instructions. Drain and set aside.
In a mixing bowl, combine ricotta cheese, shredded mozzarella, grated Parmesan, chopped spinach, garlic powder, onion powder, salt, and black pepper. Mix well.
Fill each cooked pasta shell with the cheese and spinach mixture.
Spread 1 cup of marinara sauce on the bottom of a baking dish.
Place the filled shells in the baking dish and cover with the remaining marinara sauce.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 5 minutes.
Let it cool for a few minutes before serving.