Olympus E-620 has a 12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic III+ processor. On the other hand, Panasonic GX8 has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor.
Panasonic GX8's sensor provides 8MP more than Olympus E-620's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the E-620 and GX8 sensor size comparison.
Olympus E-620 and Panasonic GX8 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-620 has 65% larger pixel area compared to Panasonic GX8, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Olympus E-620 and Panasonic GX8 sensors have been tested by DxO and the results show that GX8 has a better overall score of 75, 20 points higher compared to E-620's score of 55.