Preheat oven to 350 degrees.
Melt butter or margarine, cream cheese, and milk together in a small saucepan over medium heat until smooth and uniform. Or you can do this in the microwave (about five minutes on 50% power, stir, and two more minutes at 50% if necessary).
Place corn in a large, 9" x 13" casserole dish. Pour cream cheese mixture over the top. Season to taste with garlic salt, salt, and pepper. Gently stir all ingredients together in the casserole until just blended.
Top with shredded cheese.
Bake in oven until hot throughout and bubbly (about 25 minutes).