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 GH3 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine VII FHD processor.
Below you can see the GX8 and GH3 sensor size comparison.
Panasonic GX8 and Panasonic GH3 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 GH3 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.