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. IMP + NADP(+) + NH4(+) = GMP + 2 H(+) + NADPH Homotetramer. At least two different alleles are known. Belongs to the IMPDH/GMPR family. GuaC type 1 subfamily. The N-terminus was initially (PubMed:2758468) thought to be fused with glucose-6-phosphate-dehydrogenase (G6PD) protein in vivo. However, PubMed:2570640 showed that it encodes a GMP reductase, and PubMed:1694726 showed that the chimeric protein is an artifact.