It’s listed under “other Rhone appellations” in our WSET Diploma book, given but a short few sentences of description—all of which, let’s face, it quite generic. “Lies between the Rhone and eastern Languedoc.” “Vines are grown on south-west facing slopes.” “Maximum permitted yield is 60 hL/ha.” “Most wines are good to very good.” Blah.
Costières de Nîmes is a lot more interesting than that.