Both Panasonic GH5 and Panasonic G9 have Four Thirds sized 20.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another similarity between these two cameras is that both Panasonic G9 and Panasonic GH5 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 GH5 and G9 sensor size comparison.
Panasonic GH5 and Panasonic G9 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.