Having tried many adapted lenses on the GFX, there's a big difference between "covers the 44x33 sensor" and getting good corner performance from those lenses. Indeed there are a number of lenses that cover the 44x33 sensor or get very close to it but in almost all cases, except tilt/shift lenses, the corner performance of those lenses that do cover the sensor in the corners is abysmal. In some cases the Venus Optics Laowa Magic Format converters can give those lenses better performance in the corners but there is a 1.4x teleconverter like effect to the focal length of the lens. Of course the 0.8 crop factor of the GFX buys almost all of that back (from a 35mm system perspective) leaving you with a lens with a somewhat similar field of view on the GFX's 44x33 sensor as it does on a 36x24 sensor but without AF or EXIF data. As an example of this, I use an Irix 11mm lens on my GFX-50S with the magic format converter. I do use an LCC profile in the RAW processing flow to deal with some slight color shift and corner light falloff. This leaves me with a 35mm effective field of view of 11*1.4*0.8=12.3mm.
So if your photography is the type where corner sharpness is not critical then by all means a straight adapted 35mm lens that covers the whole sensor is an option. But if your photography needs good corner performance then there are very few 35mm system lenses that hold up under scrutiny in the corners, that even includes some high end lenses like the Otus lenses or the 70-200 f/2.8L lenses that were suggested as an alternative earlier in the thread.
