Catalyzes the synthesis of cyclic-di-GMP (c-di-GMP) via the condensation of 2 GTP molecules. Cyclic-di-GMP is a second messenger which controls cell surface-associated traits in bacteria. 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 at both 28 and 37 degrees Celsius. Decreased expression of CsgB, decreased curli expression at 28 degrees Celsius.