Both Panasonic GX9 and Pentax K-S2 have 20.0 MP resolution sensors but Panasonic GX9's sensor is Four Thirds (17.3 x 13 mm ) and Pentax K-S2's sensor is APS-C (23.5 x 15.6 mm ).
Since Pentax K-S2'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 Panasonic GX9.
Another similarity between these two cameras is that both Pentax K-S2 and Panasonic GX9 sensors lack anti-alias (Low-Pass) filters. 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 K-S2 sensor size comparison.
As seen above, Pentax K-S2 has a 1.6x 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.