Panasonic GH1 has a
12.0MP Four Thirds (18.89 x 14.48 mm ) sized CMOS sensor and features Venus Engine HD processor. On the other hand, Panasonic GH5S has a
10.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine 10 processor.
One other difference between these two cameras that is worth mentioning is that Panasonic GH5S'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 GH1 and GH5S sensor size comparison.
As seen above, Panasonic GH1 has a 1.2x Larger sensor area than Panasonic GH5S. 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.