Leica Q2 has a
47.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Maestro II processor. On the other hand, Fujifilm GFX 50R has a
51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features X Processor Pro processor.
Another difference between these two cameras is that Leica Q2's sensor lacks 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 Q2 and GFX 50R sensor size comparison.
As seen above, Fujifilm GFX 50R has a 1.7x Larger sensor area than Leica Q2. 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.