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, Olympus E-M5 II has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VII processor.
Below you can see the GX8 and E-M5 II sensor size comparison.
Panasonic GX8 and Olympus E-M5 II 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 Olympus E-M5 II has 26% larger pixel area (14.12µm2 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.