Panasonic GX9 has a
20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Nikon D750 has a
24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 4 processor.
Another difference between these two cameras is that Panasonic GX9'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 GX9 and D750 sensor size comparison.
As seen above, Nikon D750 has a 3.8x Larger sensor area than Panasonic GX9. 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.