Traditional sausage gravy is thickened with a roux made with all-purpose wheat flour and butter or oil. But you can use cornstarch in place of wheat flour for a gluten-free gravy. To thicken one cup of milk for gravy, you need one and a half teaspoons of cornstarch. The best way to add cornstarch to the pot for thickening is to stir it in a small dish with the same amount of water or milk until smooth (this is making a slurry). Then whisk or stir it into the sauce or gravy.