Panasonic S5 II has a 24.0MP Full frame (35.6 x 23.8 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Panasonic GH6 has a 25.2MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Another similarity between these two cameras is that both Panasonic GH6 and Panasonic S5 II 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 moiré occurring in certain scenes.
Below you can see the S5 Mark II and GH6 sensor size comparison.
Sensor Size and Resolution Comparison image of Panasonic S5 II and Panasonic GH6 Cameras
As seen above, Panasonic S5 II has a 3.8x Larger sensor area than Panasonic GH6. 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.