I know of two methods:
Reduce the juices (heat until a lot of the water evaporates) and then add a few pats of butter.
Add a "primal starch" thickener such as potato starch, arrowroot, or tapioca flour, if you're not averse to a little bit of additional starch. I haven't used any of these in making a gravy, so I don't know how well they work. It might be a good idea to temper them instead of just dumping straight into the gravy.