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, Fujifilm X-H1 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features X-Processor Pro processor.
Fujifilm GFX 100S's sensor provides 78MP more than Fujifilm X-H1'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.
One other difference between these two cameras that is worth mentioning is that Fujifilm X-H1'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 100S and X-H1 sensor size comparison.
Sensor Size and Resolution Comparison image of Fujifilm GFX 100S and Fujifilm X-H1 Cameras
As seen above, Fujifilm GFX 100S has a 4.0x Larger sensor area than Fujifilm X-H1. 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.