Panasonic GX9 has a
20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Panasonic LX100 II has a
17.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor.
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 LX100 II sensor size comparison.
Panasonic GX9 and Panasonic LX100 II have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Panasonic LX100 II has 19% larger pixel area (13.37µm2 vs 11.16µm2) compared to Panasonic GX9, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.