I think also it would be interesting to share as much information as possible on lenses for GFX
Here is my (very recent) experience on the subject:
1) I have got several Fotodiox adapters for GFX : Nikon, Pentax, Mamiya and Hasselblad V (HB)
All seem to work fine and focus to infinity (it is not or not always a hard focus, but a t least there does not seem to be problems)
The Pentax and HB adapters have a mount for a tripod
They are expensive but some are big, and they must be of high precision, because they might be the weakest link in the wole chain
2) Pentax lenses are really very good:
35 MM A
45 MM FA (it seems I have a good copy)
150 MM A
120 MM macro (incredibly good as many people have said)
45-85 MM zoom (very cheap in Japan and quite good on first impression)
A useful link for pentax 645 lenses:
http://forum.mflenses.com/userpix/20118/big_517_majh197_1.jpg
In 1985 Modern Photography made measures on 45 mm 50mm 75mm 150mm and 80-160mm zoom
The best lenses at f8 were measured at 70 lines pair/mm which is really quite good
I made a quick computation and the sensor of the GFX seems to be at 70-80 line pairs/mm
3) Nikon PC lenses (at least 28 mm 3.5 and probably 35 mm 2.8 as well as Canon equivalents) are cheap an cover the area of the 44x33 mm sensor
Very good buy for a cheap wide angle, fully manual of course
4) Hasselblad V lenses are also very good
I tested
a) Planar 100mm 3.5 : really very good, different color rendition that Fuji 63mm lens
Better at f8 than at f3.5
b) Sonnar 250mm 5.6 : suprisingly good at full aperture
(this means that Sonnar 150mm and 180mm are also very good since they are better optically than the 250mm, maybe not at full aperture)
c) Same lenses with 2X Mutar : not very good, somewhat disappointing
5) Mamiya lenses are somewhat disappointing, even if 110 mm 2.8 is interesting (and 80mm f4 should be good)
300 mm (at least my copy) provides little contrast
6) Manual focus is less a problem than I thought
> you just prefocus approximately
> and then move a little bit your focus left and right and look in the viewfinder for the sharpest (or the most coloured, depending on the MF aid you chose) image
> when you are convinced to have reached the best possible situation you shoot
Obviously, especially with telelenses:
> better be at f8, f11 or f16
> take several (or many) shots repeating each time the procedure and chose the best shot quietly at home
Since you need a fast shutter speed with telelenses, the ISO setting is used as the adjustement parameter to make all this possible, and ISO between 1000 and 2500 is not uncommon (ISO 400 is not uncommon also with a bright sun)
Hope this helps
Please ask for more information if you need
Guy