Catalyzes the synthesis of the second messenger cGMP from GTP (PubMed:10747978, PubMed:11703675, PubMed:18452584). Regulates cGMP production in gametocytes; however, is dispensable for the initiation of gametogenesis (PubMed:18452584). Does not have adenylate cyclase activity (PubMed:10747978). GTP = 3',5'-cyclic GMP + diphosphate Binds 2 magnesium ions per subunit (By similarity). Is also active with manganese (in vitro) (Probable). Basal guanylate activity of the recombinant guanylate cyclase domains 1 and 2 is not modulated by an increase in Ca(2+) levels or by the gametogenesis inducer xanthurenic acid. 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 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.