Sony A7S II has a 12.0MP Full frame (35.6 x 23.8 mm ) sized CMOS sensor and features Bionz X processor. On the other hand, Fujifilm GFX 100S has a 102.0MP Medium format (44 x 33 mm ) sized BSI-CMOS sensor and features X-Processor 4 processor.
Fujifilm GFX 100S's sensor provides 90MP more than Sony A7S 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.
Below you can see the A7S II and GFX 100S sensor size comparison.
As seen above, Fujifilm GFX 100S has a 1.7x Larger sensor area than Sony A7S 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.