Grease the insert of a 6-quart slow cooker with non-stick cooking spray.
Layer ⅓ of the hashbrowns on the bottom, then season with salt and pepper. Top with ⅓ of the cooked sausage, then ⅓ each of the green onions and cheddar cheese. Repeat the layers two more times, ending with the cheese.
In a large bowl, whisk together the eggs, milk, garlic powder, salt and pepper. Pour the egg mixture over the top of the hash brown, sausage and cheese layers in the slow cooker. Cook on low for 6 to 8 hours (the edges will brown). Leftovers can be stored in the refrigerator for up to 4 days.