I don't recommend adding flour straight into a cooked gravy or it will go lumpy. Instead I suggest cooking the gravy a little longer and reducing and thicken it that way. If that doesn't work, and you still want to add extra flour, then mix some into a small amount of cold water to form a smooth paste. Then add that paste to the gravy a little at a time, whisking constantly to avoid any lumps from forming. You can use flour or cornstarch this thicken the gravy this way.