Panasonic FZ300 has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Venus Engine processor. On the other hand, Panasonic GX85 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor.
Below you can see the FZ300 and GX85 sensor size comparison.
As seen above, Panasonic GX85 has a 8.0x Larger sensor area than Panasonic FZ300. 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.