Catalyzes the conversion of (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate to cyclic pyranopterin monophosphate (cPMP). 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 (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate = cyclic pyranopterin phosphate + diphosphate Cofactor biosynthesis; molybdopterin biosynthesis. The N-terminal domain of the MobA region determines nucleotide recognition and specific binding, while its C-terminal domain determines the specific binding to the target protein. In the N-terminal section; belongs to the MoaC family. In the C-terminal section; belongs to the MobA family.