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. Belongs to the IMPDH/GMPR family. GuaC type 1 subfamily.