Both Fujifilm GFX 50S and Pentax 645Z have Medium format sized 51.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
One other difference between these two cameras that is worth mentioning is that Pentax 645Z's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the GFX 50S and 645Z sensor size comparison.
Sensor Size and Resolution Comparison image of Fujifilm GFX 50S and Pentax 645Z Cameras
Fujifilm GFX 50S and Pentax 645Z have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Fujifilm GFX 50S
or the
Pentax 645Z ,
including hyperfocal distance.
