Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or oil. Allow the hash browns to thaw slightly, if needed.
In a large skillet over medium heat, cook the breakfast sausage until browned and crumbled. Add the chopped onion and diced bell pepper to the skillet and sauté until softened. Drain any excess grease.
Spread the thawed hash browns evenly in the prepared baking dish. Sprinkle the cooked sausage and vegetable mixture over the hash browns. Top with shredded cheddar cheese.
In a bowl, whisk together the eggs, milk, salt, pepper, garlic powder, and onion powder until well combined. Pour this egg mixture evenly over the hash browns, sausage, and cheese in the baking dish.
Bake in the preheated oven for 45-55 minutes or until the center is set, and the top is golden brown. Allow the casserole to cool for a few minutes before slicing. Garnish with fresh chopped parsley if desired. Serve warm, and enjoy your delicious gluten-free Sausage Hashbrown Breakfast Casserole!