Catalyzes the irreversible NADPH-dependent deamination of GMP to IMP. It functions in the conversion of nucleobase, nucleoside and nucleotide derivatives of G to A nucleotides, and in maintaining the intracellular balance of A and G nucleotides. {ECO:0000255|HAMAP- Rule:MF_03195}. This is the function of Ensembl gene identifier ENSG00000137198 (GMPR, guanosine monophosphate reductase).