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