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 Activated by GTP and inhibited by ATP and IMP. Mycophenolic acid (MPA) is a competitive inhibitor of the enzyme with respect to NADPH. kcat is 0.059 min(-1) with GMP as substrate and 0.043 min(-1) with NADPH as substrate. Homotetramer. Belongs to the IMPDH/GMPR family. GuaC type 1 subfamily. Extended N-terminus.