Pentax 645Z has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features PRIME III processor. On the other hand, Fujifilm GFX 100 has a 99.9MP Medium format (44 x 33 mm ) sized BSI-CMOS sensor and features X-Processor 4 processor. Fujifilm GFX 100's sensor provides 48.9MP more than Pentax 645Z's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Another difference between these two cameras is that Pentax 645Z's sensor lacks 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 moire occurring in certain scenes.
Below you can see the 645Z and GFX 100 sensor size comparison.
Pentax 645Z and Fujifilm GFX 100 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Pentax 645Z has 95% larger pixel area compared to Fujifilm GFX 100, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.