Panasonic GH3 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine VII FHD processor. On the other hand, Fujifilm GFX 50S has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features X Processor Pro processor.
Fujifilm GFX 50S's sensor provides 35MP more than Panasonic GH3'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 GH3 and GFX 50S sensor size comparison.
As seen above, Fujifilm GFX 50S has a 6.5x Larger sensor area than Panasonic GH3. 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.