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 GX8 has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor. Panasonic GX8's sensor provides 10MP more than Panasonic GH5S's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
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 moire occurring in certain scenes.
Below you can see the GH5S and GX8 sensor size comparison.
Panasonic GH5S and Panasonic GX8 have 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 98% larger pixel area compared to Panasonic GX8, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.