I've test the CONTAX 35 PC... and wasn't impress at all...
I've seen some sample made with the Pantax D, not perfect also...
Do you think that using the Contax 645 35mm fixed at f11 with a shift adaptor could be better ?
Of course, the C645 35mm lens isn't perfect, either. But if you like the Zeiss look -- and I certainly do! -- it's the only option available at that focal length...
What subject matter will you be photographing and how will you use the movements?
I ask, because I have a C645 35mm lens that I use as a fixed-aperture shift lens on my A7R for photographing urban and suburban street and alley scenes, which often include a significant architectural element in the compositions, and it will display a small amount of "mustache" distortion that is difficult to eliminate entirely even when using post-exposure correction software (including Alpa's correction software and profile, alas.)
While I'm usually okay with this, because no lens is perfect and I very much like the performance of this lens otherwise, if you use the lens to stitch a larger / wider file instead of to apply rise / fall movements to correct for keystoning, you may sometimes see very small amounts of ripple in straight lines that run across the frame from side-to-side and/or slightly lower resolution along the stitch lines, because the software has to massage the pixels a bit in order to align the files.
For the most part, I'm okay with this, too, because I'm not really photographing the structures in the photo but the scene overall, so rendering the structures with geometric perfection isn't my primary goal. But some photographers -- and, I suspect, clients -- are not, hence you should at least be aware of this effect.
On my sample, at least, the image quality holds up surprisingly well out to very near the edge of the image circle -- see, for example, the structure at the top of this grain elevator photographed with ~19 mm of rear rise, which is about the maximum possible on my A7R before the vignetting becomes visible, as it's just starting to do here:
Here's another grain elevator photo, taken with slightly less rear rise -- ~17 mm, IIRC -- hence less vignetting is visible:
As I wrote, even though using the C645 lenses with fixed apertures is a PITA (because I photograph a lot at night, so I have to compose and focus wide-open, then remove the lens to stop down the aperture [using a small A5100 body / Fringer / NAM-1 adapter combo I carry with me for this purpose] then remount it on my modified FrankenKameras [i.e., fancy shift and tilt/shift lens adapters I've made from other cameras] without altering its focus [which I sometimes end up having to tweak slightly afterward regardless] or even worse, dropping it), the results are usually worth the additional effort, especially as these lenses can be purchased quite affordably these days.
That said, YMMV, because the GFX sensor is larger than my A7R's sensor, hence your range of acceptable rise / fall / shift movements with it will be proportionally shorter than mine. Alas, I am not aware of anyone else who is using a GFX body with this lens for shift movements, so you may well have no choice but to boldly go where no photographer has gone before...