Any particular reason to believe that the G4003G (gunsmith) lathe would be different from the G4003 ? Per their respective manuals, both include a mounted 86/91 idler, a mounted 60T change gear, and the same selection of accessory change gears. The metric threading charts in the manuals appear to be the same.
Yes. It looked to me on casual inspection of the respective manuals that on the G4003G either of the F and G gears could engage either of the 86 or 91 tooth idler (that is, 4 possible combinations), while on the G4003 that F was always on 91 while G could engage either 86 or 91 (that is, 2 possible combinations).
On the Enco and Harbor Freight versions there is no step on the idler, just one possible engagement 80T for both F and G, that is, a unity ratio.
Thus the Enco/HF idler provides no additional possible ratios, the G4003 idler doubles them, and the G4003G quadruples them.