Panasonic GX8 has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Panasonic G85 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Another difference between these two cameras is that Panasonic G85'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 GX8 and G85 sensor size comparison.
Panasonic GX8 and Panasonic G85 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 G85 has 27% larger pixel area (14.21µm2 vs 11.16µm2) compared to Panasonic GX8, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.