Catalyzes the phosphorylation of GMP to GDP. Essential enzyme for recycling GMP and indirectly, cyclic GMP (cGMP). Involved in the cGMP metabolism in photoreceptors. ATP + GMP = ADP + GDP Up-regulated by RD3. Monomer (PubMed:8097461). Interacts with RD3 (By similarity). Colocalizes with RD3 in photoreceptor inner segments and to a lesser extent in the outer plexiform layer. Belongs to the guanylate kinase family.