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. Induced by 0.3M NaCl in an RpoS-dependent fashion. Deletion of the gene increases swimming motility and early biofilm formation. Has a EGEVF motif instead of the conserved GG[D/E]EF motif characteristic of active diguanylate cyclases, suggesting that CdgI may be inactive. However, site-directed mutagenesis shows that it is probably a functional diguanylate cyclase.