Drain the potatoes and transfer them to a large mixing bowl. Add the butter, mayonnaise, sour cream, milk, and half of the cheese to the potatoes. Add the garlic salt and pepper. Use an electric mixer to whip the potato mixture until thoroughly combined and spoon into the prepared baking dish. Sprinkle the crumbled bacon and the remaining cheese over the top and bake for 20 minutes, or until the cheese is melted and the casserole is heated through.