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 (By similarity). Localizes at the cell poles through interaction with FimV where it increases the local pools of c-di-GMP (PubMed:32080219). 2 GTP = cyclic di-3',5'-guanylate + 2 diphosphate Binds 1 Mg(2+) ion per monomer. Purine metabolism; 3',5'-cyclic di-GMP biosynthesis. Localizes at the cell poles in a FimV-dependent manner.