Fujifilm GFX 50R has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features X Processor Pro processor. On the other hand, Panasonic S1R has a 47.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Venus Engine processor.
Another difference between these two cameras is that Panasonic S1R'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 GFX 50R and S1R sensor size comparison.
As seen above, Fujifilm GFX 50R has a 1.7x Larger sensor area than Panasonic S1R. 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.