Participates in processes of transmission and amplification of the visual signal. cGMP-PDEs are the effector molecules in G-protein-mediated phototransduction in vertebrate rods and cones. 3',5'-cyclic GMP + H2O = GMP + H(+) Tetramer composed of two catalytic chains (alpha and beta), and two inhibitory chains (gamma). The C-terminal region is important in conferring inhibition. Belongs to the rod/cone cGMP-PDE gamma subunit family.