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 (PubMed:12009299, PubMed:12669231, PubMed:16359702, PubMed:22037469). Plays a role in modulating cellular differentiation (PubMed:12669231). IMP + NADP(+) + NH4(+) = GMP + 2 H(+) + NADPH Homotetramer. Highly expressed in heart, skeletal muscle, kidney, brain, liver, prostate, spleen, placenta, testis and ovary. Low expression in colon, thymus and peripheral blood leukocytes. Belongs to the IMPDH/GMPR family. GuaC type 1 subfamily.