Catalyzes the phosphorylation of GMP to GDP. Essential enzyme for recycling GMP and indirectly, cyclic GMP (cGMP) (PubMed:31201273). Involved in the cGMP metabolism in photoreceptors (By similarity). It may also have a role in the survival and growth progression of some tumors (PubMed:31201273). In addition to its physiological role, GUK1 is essential for converting prodrugs used for the treatment of cancers and viral infections into their pharmacologically active metabolites, most notably acyclovir, ganciclovir, and 6-thioguanine and its closely related analog 6-mercaptopurine (PubMed:197968, PubMed:6248551, PubMed:6306664). {ECO:0000250|UniProtKB:P46195, ECO:0000269|PubMed:197968, ECO:0000269|PubMed:31201273, ECO:0000269|PubMed:6248551, ECO:0000269|PubMed:6306664}. This is the function of Ensembl gene identifier ENSG00000143774 (GUK1, guanylate kinase 1).