Purine salvage pathway enzyme that catalyzes the transfer of the ribosyl-5-phosphate group from 5-phospho-alpha-D-ribose 1-diphosphate (PRPP) to the N9 position of the 6-oxopurines guanine and xanthine to form the corresponding ribonucleotides GMP (guanosine 5'-monophosphate) and XMP (xanthosine 5'-monophosphate), with the release of PPi. To a lesser extent, also acts on hypoxanthine. diphosphate + GMP = 5-phospho-alpha-D-ribose 1-diphosphate + guanine diphosphate + XMP = 5-phospho-alpha-D-ribose 1-diphosphate + xanthine diphosphate + IMP = 5-phospho-alpha-D-ribose 1-diphosphate + hypoxanthine Purine metabolism; GMP biosynthesis via salvage pathway; GMP from guanine: step 1/1. Purine metabolism; XMP biosynthesis via salvage pathway; XMP from xanthine: step 1/1. Homotetramer. Belongs to the purine/pyrimidine phosphoribosyltransferase family. XGPT subfamily.