Fujifilm GFX 50S has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features X Processor Pro processor. On the other hand, Nikon D810A has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features EXPEED 4 processor. Fujifilm GFX 50S's sensor provides 15MP more than Nikon D810A'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.
Below you can see the GFX 50S and D810A sensor size comparison.
As seen above, Fujifilm GFX 50S has a 1.7x Larger sensor area than Nikon D810A. 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.