Catalyzes the phosphorylation of GMP to GDP. Essential enzyme for recycling GMP and indirectly, cyclic GMP (cGMP) (PubMed:8243671, PubMed:29515371, PubMed:7911663). Involved in the cGMP metabolism in photoreceptors (PubMed:29515371, PubMed:8243671). ATP + GMP = ADP + GDP Up-regulated by RD3. Monomer (By similarity). Interacts with RD3 (By similarity). Colocalizes with RD3 in photoreceptor inner segments and to a lesser extent in the outer plexiform layer. Expressed in retina. Belongs to the guanylate kinase family.