Both Fujifilm GFX 50R and Canon 5DS R have 51.0 MP resolution sensors but Fujifilm GFX 50R's sensor is Medium format (44 x 33 mm ) and Canon 5DS R's sensor is Full frame (36 x 24 mm ).
Since Fujifilm GFX 50R'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.
One other difference between these two cameras that is worth mentioning is that Canon 5DS R'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 50R and 5DS R sensor size comparison.
As seen above, Fujifilm GFX 50R 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.