Leica SL2 has a 47.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Maestro III processor. On the other hand, Fujifilm GFX 50S has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features X Processor Pro processor.
Another difference between these two cameras is that Leica SL2'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 SL2 and GFX 50S sensor size comparison.
As seen above, Fujifilm GFX 50S has a 1.7x Larger sensor area than Leica SL2. 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.