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, Sony A6600 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz X processor.
Fujifilm GFX 50R's sensor provides 27MP more than Sony A6600'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 A6600 sensor size comparison.
Sensor Size and Resolution Comparison image of Fujifilm GFX 50R and Sony A6600 Cameras
As seen above, Fujifilm GFX 50R has a 4.0x Larger sensor area than Sony A6600. 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.