To make this Thanksgiving Leftover Casserole Recipe gluten-free, use gluten-free stuffing, gluten-free gravy, and certified gluten-free chicken broth. Confirm that the mashed potatoes, cranberry sauce, turkey, and milk are free from gluten-containing additives or cross-contact. If the turkey was prepared with stuffing inside the cavity or seasoned with gluten-containing ingredients, use a separate gluten-free turkey portion instead. Prepare and bake the casserole with clean utensils, cookware, and surfaces to prevent cross-contamination.