Transfers a GMP moiety from GTP to Mo-molybdopterin (Mo-MPT) cofactor (Moco or molybdenum cofactor) to form Mo-molybdopterin guanine dinucleotide (Mo-MGD) cofactor. GTP + H(+) + Mo-molybdopterin = diphosphate + Mo-molybdopterin guanine dinucleotide The N-terminal domain determines nucleotide recognition and specific binding, while the C-terminal domain determines the specific binding to the target protein. Belongs to the MobA family.