I think the best way is to bring whatever liquid you're making gravy from to a boil over high heat, adding whipping cream and letting it boil away until it thickens. You can also add a bit of cream cheese for its thickening power.

FYI, these are the ECC's on thickeners per Tbsp.:

All purpose flour - 5.75
Arrowroot - 6.78
Cornstarch - 7.23

Karen