Monoterpene synthase which catalyzes the conversion of geranyl diphosphate to gamma-terpinene and the minor products limonene, alpha-pinene, beta-pinene, alpha-terpinolene, alpha-thujene, alpha-terpinene, myrcene and sabinene. (2E)-geranyl diphosphate = diphosphate + gamma-terpinene Manganese > magnesium. Inhibited by 100 mM KCl. Optimum pH is 7.0. Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.