Preheat Oven: Preheat your oven to 375°F (190°C).
Prepare the Turkey Mixture: In a large mixing bowl, combine the shredded turkey and gravy. Season with garlic powder, onion powder, salt, and pepper. Stir until well mixed.
Assemble the Casserole: In a greased casserole dish, spread the turkey mixture evenly at the bottom. If you're using sour cream, stir it into the mashed potatoes for extra creaminess, and then spread the mashed potatoes over the turkey layer.
Top with Cheese: Sprinkle the shredded cheese over the mashed potatoes.
Bake: Place the casserole in the oven and bake for 25-30 minutes, or until the cheese is melted and the top is golden and bubbly.
Serve: Let the casserole cool for a few minutes before serving.