Non-catalytic subunit of the N-acetylglucosamine-1-phosphotransferase complex, an enzyme that catalyzes the formation of mannose 6-phosphate (M6P) markers on high mannose type oligosaccharides in the Golgi apparatus. Binds and presents the high mannose glycans of the acceptor to the catalytic alpha and beta subunits (GNPTAB). Enhances the rate of N-acetylglucosamine-1-phosphate transfer to the oligosaccharides of acid hydrolase acceptors. Homodimer; disulfide-linked. Hexamer of two alpha (GNPTAB), two beta (GNPTAB) and two gamma (GNPTG) subunits; disulfide-linked. The alpha and/or the beta subunits of the enzyme constitute the catalytic subunits (By similarity). Widely expressed. Highly expressed in the liver, intestine, brain, thymus, testis and ovary. Cys-245 mediates the formation of the interchain disulfide bond for formation of the homodimer. Cys-142, Cys-157 and Cys-169 are involved in intramolecular disulfide bonds formation (PubMed:21173149).