Hello Rollsmann44,
i have both cameras the 645Z and the GFX. With ISO 1600 the cameras take practically nothing.
The GFX is better what the flashes with manual lenses if one wants to move out of the flash synchronous time (1/125s) (HSS or SyperSync). For both cameras then a TTL compatible device must be recognized by the camera. In most cases, a flash trigger. The 645Z would like to calculate the flash power automatically via TTL. For this purpose, it needs a minimum aperture, shutter speed and distance. If the aperture is missing, as with manual lenses without transmission, the 645Z denies the release of the flash. The GFX does not do that. It accepts that one can use the TTL signal also only partially as a control signal. The GFX also releases the flash without a TTL device outside the flash sync time. That goes with me to 1/160s without which a shading in the picture or the curtain is recognizable.
Otherwise the operating concept is different for both cameras.
In the GFX interferes me personally that there are no memory banks for the complete presets of the camera and the camera does not notice where I was last in the menu.
The 645z I can use somewhat better with gloves than the GFX.
The angle view finder on the GFX is a real pound in the weighing pan.
If you want to buy/adapt different manual medium format lenses, the GFX is actually the first choice. I use Pentax 67, Pentax 645 and Mamiya 645 lenses via adapter directly to the GFX. The image quality is very good, depending on the lens and imaging performance. From KB lenses at the GFX, I am not so enthusiastic (except for a few exceptions). This is not so much about the shading, but rather about the rapidly declining imaging performance towards the edge.
Otherwise the GFX is the more modern system. Pentax unfortunately has long been nothing for the 645z announced and delivered. What I find very unfortunate and I can at present not form a correct opinion where it could go in the future with Pentax and medium format. With us in Germany, the Fuji Professional Service is very good. The Pentax 645 service is unfortunately worse in terms of spare parts supply and loan equipment. In your region, however, it might look different.
Greeting Gerd