Catalyzes the synthesis of the second messenger cGMP from GTP (Probable). In asexual blood stage schizonts, required for cGMP production which is essential for PKG activation, PKG-dependent Ca(2+) release, and ultimately merozoite egress from host erythrocytes (PubMed:33500341). GTP = 3',5'-cyclic GMP + diphosphate Binds 2 magnesium ions per subunit (By similarity). Is also active with manganese (in vitro) (By similarity). In gametocytes, localizes to the cell membrane and/or the parasitophorous vacuole membrane. During the parasite blood stage, highly expressed in mature schizonts and in gametocytes. The N-terminus contains a P-type ATPase-like domain which may be important for guanylate cyclase activity. Knockout at the asexual blood stage causes growth arrest (PubMed:33500341). Mature schizonts accumulate in host erythrocytes due to a defect in merozoite egress (PubMed:33500341). Lack of swelling or rupture of the parasitophorous vacuole membrane and, no Ca(2+) release from intracellular stores and no secretion of protease SERA5 (PubMed:33500341). Mature schizonts fail to synthesize cGMP; no defect in cAMP levels (PubMed:33500341). 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. Unlike the two guanylate cyclase domains of GCbeta, the guanylate cyclase domains 1 and 2 of GCalpha lack catalytic activity when expressed on their own or in combination.