You can thicken gravy by reducing and simmering it.
One of the simplest ways to thicken your gravy is to let it cook longer.
Add cornstarch: To thicken your gravy with cornstarch, make a slurry, which is a liquid-based paste.
Use a beurre manié: French for “kneaded butter,” beurre manié is a method of thickening gravy that is similar to a roux.