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, Nikon D810 has a
36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features EXPEED 4 processor.
Fujifilm GFX 50R's sensor provides 15MP more than Nikon D810'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 Nikon D810'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 D810 sensor size comparison.
As seen above, Fujifilm GFX 50R has a 1.7x Larger sensor area than Nikon D810. 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.