Panasonic GH5S has a
10.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine 10 processor. On the other hand, Panasonic G2 has a
12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine HD II processor.
Another difference between these two cameras is that Panasonic GH5S'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 GH5S and G2 sensor size comparison.
Panasonic GH5S and Panasonic G2 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 GH5S has 18% larger pixel area compared to Panasonic G2, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.