Prepare a 13" x 9" pan with cooking spray. Arrange six stale bread slices into bottom of pan. Brush with one tablespoon egg whites. Sprinkle with half turkey, stuffing, and cheese. Brush remaining bread with remaining one tablespoon egg whites and place, egg white side-down, on cheese. Sprinkle with remaining turkey, stuffing, and cheese; set aside. In a mixing bowl, combine the remaining egg whites, milk, gravy, salt, and black pepper. Beat well. Pour over strata, allowing to soak in. Cover tightly with foil and refrigerate for four hours, or until egg white mixture has been absorbed by the bread slices. Preheat oven to 350. Bake strata, uncovered, 50 minutes, or until puffed and golden.