Pour pan drippings into a large measuring cup. Skim and reserve fat from pan drippings. Measure fat, and add melted margarine or butter, if necessary, to make 1/2 cup. Add reserved giblet broth to the skimmed drippings to measure 2-1/2 cups total; set aside. Pour the 1/2 cup of fat into a medium saucepan (discard any remaining fat). Stir in flour, salt, and pepper. Cook and stir over medium heat for 2 minutes. Gradually whisk giblet broth mixture into flour mixture. Cook and stir until thickened and bubbly. Add milk. Cook and stir 1 minute more. Add sherry, if desired. Garnish with snipped fresh herbs, if desired. Serve immediately.