Catalyzes the synthesis of cyclic-di-GMP (c-di-GMP) via the condensation of 2 GTP molecules. 2 GTP = cyclic di-3',5'-guanylate + 2 diphosphate Binds 1 Mg(2+) ion per monomer. Purine metabolism; 3',5'-cyclic di-GMP biosynthesis. Homodimer. Expressed during exponential and post-exponential growth, it is slightly more highly expressed at 37 than 28 degrees Celsius and is more highly induced on solid medium. Deletion of the gene increases swimming motility (PubMed:21181144). Disruption of this gene partially suppresses the reduced motility of a pdeH disruption. Full suppression i.e. wild-type motility, requires concomitant disruption of dgcJ, dgcE and dgcQ (PubMed:18765794).