Both Canon G9 X II and Panasonic G9 have 20.0 MP resolution sensors but Canon G9 X II's sensor is 1″ (13.2 x 8.8 mm ) and Panasonic G9's sensor is Four Thirds (17.3 x 13 mm ).
Since Panasonic G9's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Canon G9 X II.
Another difference between these two cameras is that Panasonic G9'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 moire occurring in certain scenes.
Below you can see the G9 X II and G9 sensor size comparison.
As seen above, Panasonic G9 has a 1.9x Larger sensor area than Canon G9 X II. 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.