Panasonic GH6 has a
25.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor . On the other hand, OM System OM-1 has a
20.0MP Four Thirds (17.4 x 13 mm ) sized Stacked BSI Live MOS Sensor sensor .
Panasonic GH6's sensor provides 5MP more than OM System OM-1'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 similarity between these two cameras is that both OM System OM-1 and Panasonic GH6 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 GH6 and OM-1 sensor size comparison.
Panasonic GH6 and OM System OM-1 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.