Magnesium-dependent guanylyl cyclase that catalyzes the formation of guanosine 3',5'-cyclic monophosphate (cGMP) from guanosine 5'-triphosphate (GTP) (PubMed:12482758). Can also use ATP as substrate with a low activity (PubMed:12482758). GTP = 3',5'-cyclic GMP + diphosphate Nucleotide metabolism. Functions both as monomer and homooligomer.