Preheat the oven to 375°F (190°C).
Prepare a baking dish by spraying it with non-stick cooking spray.
In a large bowl, combine the diced potatoes, shredded chicken, cooked bacon, ranch dressing, garlic powder, salt, and pepper.
Transfer the mixture to the prepared baking dish and spread it evenly.
Top the mixture with shredded cheese and chopped green onions.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 15-20 minutes, until the cheese is bubbly and golden.
Remove the casserole from the oven and let it cool for a few minutes before serving.