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, Canon 6D MII has a 26.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features DIGIC 7 processor. Fujifilm GFX 50R's sensor provides 25MP more than Canon 6D MII'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 50R and 6D MII sensor size comparison.
As seen above, Fujifilm GFX 50R has a 1.7x Larger sensor area than Canon 6D MII. 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.