May have a role in phototransduction (PubMed:8095978). Catalyzes the conversion of GTP to cGMP, a common second messenger that is utilized in a wide variety of cells and signal transduction pathways (PubMed:7797526). A second subunit is required for enzyme activity (PubMed:7797526). GTP = 3',5'-cyclic GMP + diphosphate Heterodimer. Head, where it is preferentially expressed in the CNS and the retina (PubMed:8095978, PubMed:7797526). Not found in bodies (PubMed:8095978). Belongs to the adenylyl cyclase class-4/guanylyl cyclase family.