Involved in the purine-salvage pathway. Catalyzes the NADPH-dependent conversion of GMP to IMP. IMP + NADP(+) + NH4(+) = GMP + 2 H(+) + NADPH Purine metabolism; IMP biosynthesis via salvage pathway. Belongs to the IMPDH/GMPR family. GuaB1 subfamily. Truncated N-terminus.