Is there a Mamiya/Phase One to Fuji Kipon T/S adapter? Mamiya makes a 28mm lens in leaf-shutter and non leaf-shutter versions. It's expensive at $5500 without the leaf shutter, $6000 with (the leaf-shutter version is listed as a Schneider-Kreuznach and the other is listed as a Phase One - not sure if they're optically different). Used versions show up for about half that, generally branded Mamiya. There's also a Hasselblad H system 28mm around the same price (which would, of course, require a Hasselblad to Fuji adapter). The Hasselblad lens is probably a Fujinon underneath it all. The Hasselblad lens is digital-only, covering a sensor slightly larger than the GFX sensor, but not 645 film - shift movements might be somewhat limited. The Mamiya/Schneider/Phase lens is listed as actually covering 645 film - it should have plenty of movements on a GFX.
The other alternative would be a technical camera or "baby" view camera with a digital back. I believe there are technical camera lenses that wide (they look vaguely like view camera lenses, although many of them have a focusing helicoid depending on the design of the camera, and they don't cover 4x5"), and (since the back has no flange distance at all), focusing and movements are easier than with a technical adapter on a Fuji. This is almost certainly even more expensive than a Mamiya lens (a used back new enough to have reasonable live view will be close to $10,000 from a dealer before you add a tech camera and a lens, although they're sometimes cheaper on the forum here).