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 (PubMed:20303158). 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. Constitutively expressed at both 28 and 37 degrees Celsius. Induced by 0.3 M NaCl. Expression is RpoS dependent. Deletion of the gene increases swimming motility and early biofilm formation (PubMed:21181144). Disruption partially suppresses the reduced motility of a pdeH disruption; concomitant disruption of dosC, dgcE, dgcQ and dgcN completely restores motility, suggesting these 4 genes, together with the c-di-GMP phosphodiesterase PdeH, form a network that regulates cell motility by altering levels of c-di-GMP (PubMed:20303158).