Catalyzes the conversion of (2Z,6E)-farnesyl diphosphate (FPP) to yield the bicyclic sesquiterpene (1S,6S,7R)-(-)-gamma-cadinene via a 1,10-cyclization, which requires the abstraction of the pyrophosphate from FPP to yield a germacradienyl cation (PubMed:23307484, PubMed:27666571, PubMed:27829890). The only accepted substrate is (2Z,6E)-farnesyl diphosphate (FPP) (PubMed:27829890). (2Z,6E)-farnesyl diphosphate = (-)-gamma-cadinene + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Asp-Xaa-Xaa-Asp (DDXXD) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.