Arrange potatoes in the bottom of an 9×13-inch baking dish. Sprinkle with cheese, ham, and green onion.
In a large mixing bowl, combine eggs, evaporated milk, garlic salt, and pepper. Pour evenly over ingredients in baking dish.
Cover and refrigerate up to overnight, if desired. If chilled, bake uncovered until center is set, about 1 hour 30 minutes. If baking unchilled, reduce time to 1 hour 15 minutes.