Sony A9 has a 24.0MP Full frame (35.6 x 23.8 mm ) sized BSI-CMOS sensor and features BIONZ X processor. On the other hand, Panasonic G9 has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Another similarity between these two cameras is that both Panasonic G9 and Sony A9 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 moire occurring in certain scenes.
Below you can see the A9 and G9 sensor size comparison.
As seen above, Sony A9 has a 3.8x Larger sensor area than Panasonic G9. 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.