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-400 has a
10.0MP Four Thirds (17.3 x 13 mm ) sized CCD sensor .
Panasonic GX8's sensor provides 10MP more than Olympus E-400'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 GX8 and E-400 sensor size comparison.
Panasonic GX8 and Olympus E-400 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.