Upload & Sell: Off
Contax G actually has a longer registration distance than Leica M, and there are now a number of Leica M helicoid adapters (some for as little as $35. Personally, I'd pay as much as $90 for such an adapter)
If I did the m-conversion, I'd have to do it for all 3 of my G lenses, which would get pricey.
the problem is you need to fit both the gearing for the focus drive and the helicoid in a small space, the extra 2mm or whatever it is won't be much help. also, those cheap helicoid adapters have plastic helicoids and tend to wobble with any kind of torque. i wouldn't trust them to be able to take a flat field picture. my slightly more expensive one always induces a small amount of tilt. in order to make it work you would have to either use higher grade metals and fine machining (expensive) or expand the diameter a great deal, which would probably interfere with the grip. i don't think a $90 version of such an adapter would be worth buying. what might actually work better (and would be less fiddly to use) would be a straight up contax g to nex helicoid adapter with nothing on it to interface with the lens's own helicoid. it would be less fiddly with only one ring and the helicoid would be smoother than the geared screw drive thing on current contax g adapters. there is enough difference in the flange distances to allow infinity to reasonably close focus on all the lenses except the 90mm.