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