Involved in the purine-salvage pathway (PubMed:35338694). Catalyzes the NADPH-dependent conversion of GMP to IMP (PubMed:35338694). Has no inosine-5'-monophosphate dehydrogenase (IMPDH) activity (PubMed:21081761). IMP + NADP(+) + NH4(+) = GMP + 2 H(+) + NADPH Activity is highest with Rb(+), followed by K(+), NH4(+) and Cs(+). Optimum pH is 7.6-8.2. Purine metabolism; IMP biosynthesis via salvage pathway. Homooctamer composed of two tetramers. Was identified as a high-confidence drug target. Belongs to the IMPDH/GMPR family. GuaB1 subfamily.