Converts guanine to guanosine monophosphate, and hypoxanthine to inosine monophosphate. Transfers the 5- phosphoribosyl group from 5-phosphoribosylpyrophosphate onto the purine. Works with guanine, hypoxanthine and xanthine. Plays a central role in the generation of purine nucleotides through the purine salvage pathway (By similarity).