Catalyzes the synthesis of the second messenger cGMP from GTP (By similarity). Probably by regulating cGMP production, required for ookinete gliding motility, which is necessary for the ookinete to traverse the midgut epithelium of the mosquito (PubMed:17030505, PubMed:19779564). GTP = 3',5'-cyclic GMP + diphosphate Binds 2 magnesium ions per subunit (By similarity). Is also active with manganese (in vitro) (By similarity). During the blood stage, specifically expressed in gametocytes. The N-terminus contains a P-type ATPase-like domain which is required for guanylate cyclase activity. In the mosquito, ookinetes have severe defect in motility and their ability to traverse the midgut epithelium is impaired resulting in complete developmental arrest (PubMed:17030505, PubMed:19779564). Normal development of gametocytes, normal exflagellation and fertilization, and normal development of zygote into ookinete (PubMed:19779564). In the N-terminal section; belongs to the cation transport ATPase (P-type) (TC 3.A.3) family. Type IV subfamily. In the C-terminal section; belongs to the adenylyl cyclase class-4/guanylyl cyclase family.