There is some information on here already, for example:
Without any knowledge of Fuji MF I’d still say that it would be extraordinary if reversing Fuji standard lenses would work well for what is a very demanding task, flat field, high resolution to the corners etc.
Just thinking about the numbers, if say you have the 50MP GFX then full capture of 35mm format leaves you with 45MP from a 44mm x 29mm section. If you decide to crop in and just capture at 1:1 to use 36 x 24 mm of the sensor then you get 30MP. If you do the latter then any of the manual lenses recommended on here for full frame capture should work just as well provided you can fit them via a suitable adapter but your high end camera deserves a high end lens I would have thought, the 75mm Apo-Rodagon D 1x gets a lot of praise on here and I imagine that should be fine for the full 44mm x 29mm capture even though you’ll be going slightly beyond 1:1. However your camera is physically quite large so it might be less easy, or at least quite expensive, to fit on to suitable bellows etc.