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, Fujifilm X-E3 has a
24.0MP APS-C (23.6 x 15.6 mm ) sized CMOS X-TRANS III sensor and features EXR Processor III processor.
One other difference between these two cameras that is worth mentioning is that Fujifilm X-E3's sensor doesn't have an 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 moiré occurring in certain scenes.
Below you can see the GX8 and X-E3 sensor size comparison.
As seen above, Fujifilm X-E3 has a 1.6x Larger sensor area than Panasonic GX8. 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.