Both Canon 5DS R and Fujifilm GFX 50S have 51.0 MP resolution sensors but Canon 5DS R's sensor is Full frame (36 x 24 mm ) and Fujifilm GFX 50S's sensor is Medium format (44 x 33 mm ).
Since Fujifilm GFX 50S's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Canon 5DS R.
Another difference between these two cameras is that Canon 5DS R'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 5DS R and GFX 50S sensor size comparison.
As seen above, Fujifilm GFX 50S has a 1.7x Larger sensor area than Canon 5DS R. 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.