Catalyzes the conversion of GTP to cyclic-di-GMP (c-di-GMP). Shows activity under aerobic and anaerobic reaction conditions. 2 GTP = cyclic di-3',5'-guanylate + 2 diphosphate Shows higher activity with Mn(2+) than Mg(2+). Allosterically regulated by a feedback inhibition loop. Exists as a homodimer and as larger aggregates. Both dimers and aggregates possess DGC activity.