Both Panasonic GX9 and Sony RX100 IV have 20.0 MP resolution sensors but Panasonic GX9's sensor is Four Thirds (17.3 x 13 mm ) and Sony RX100 IV's sensor is 1″ (13.2 x 8.8 mm ).
Since Panasonic GX9'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 Sony RX100 IV.
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 RX100 IV sensor size comparison.
As seen above, Panasonic GX9 has a 1.9x Larger sensor area than Sony RX100 IV. 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.