Measure pan drippings, adding some water if necessary to bring level up as desired. Return measured liquid to the roasting pan and bring to a boil over high heat on stovetop.
Meanwhile, measure cold water. Whisk cornstarch into cold liquid until smooth.
Whisk this cold slurry into the boiling drippings in pan, stirring constantly. The gravy should thicken up immediately. Turn down the heat, keep warm, and serve.
Notes
The key to smooth, gluten free gravy is to blend 1 tablespoon cornstarch per cup of total liquid smoothly into cold liquid before adding it to the hot.