Both Sony a5000 and Panasonic GX9 have 20.0 MP resolution sensors but Sony a5000's sensor is APS-C (23.2 x 15.4 mm ) and Panasonic GX9's sensor is Four Thirds (17.3 x 13 mm ).
Since Sony a5000'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.
One other difference between these two cameras that is worth mentioning is that Panasonic GX9's sensor doesn't have an 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 a5000 and GX9 sensor size comparison.
As seen above, Sony a5000 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.