Rod-specific cGMP phosphodiesterase that catalyzes the hydrolysis of 3',5'-cyclic GMP. This protein participates in processes of transmission and amplification of the visual signal. 3',5'-cyclic GMP + H2O = GMP + H(+) Binds 2 divalent metal cations per subunit. Site 1 may preferentially bind zinc ions, while site 2 has a preference for magnesium and/or manganese ions. Oligomer composed of two catalytic chains (alpha and beta), an inhibitory chain (gamma) and the delta chain. Belongs to the cyclic nucleotide phosphodiesterase family.