Panasonic S5 II has a 24.0MP Full frame (35.6 x 23.8 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Fujifilm GFX 50S II has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features X-Processor 4 processor.
Fujifilm GFX 50S II's sensor provides 27MP more than Panasonic S5 II'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 Panasonic S5 II'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 moiré occurring in certain scenes.
Below you can see the S5 Mark II and GFX 50S II sensor size comparison.
As seen above, Fujifilm GFX 50S II has a 1.7x Larger sensor area than Panasonic S5 II. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.