Place sausage in a large, deep skillet. Cook over medium-high heat until evenly brown. Remove sausage with a slotted spoon, leaving the drippings in the pan. Stir in the butter until melted. Add flour, and stir until smooth. Reduce heat to medium, and cook until light brown. Gradually whisk in milk, and cook until thickened. Season with salt and pepper, and stir in cooked sausage. Reduce heat, and simmer for 12 to 15 minutes. If gravy becomes too thick, stir in a little more milk.