Catalyzes the synthesis of cyclic-di-GMP (c-di-GMP) via the condensation of 2 GTP molecules (By similarity). Cyclic-di-GMP is a second messenger which controls cell surface-associated traits in bacteria. Involved in the regulation of cellulose production (By similarity). 2 GTP = cyclic di-3',5'-guanylate + 2 diphosphate Binds 1 Mg(2+) ion per monomer. Glycan metabolism; bacterial cellulose biosynthesis. Purine metabolism; 3',5'-cyclic di-GMP biosynthesis. Homodimer. Could be the product of a pseudogene. Truncated C-terminus.