Panasonic GF8 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Nikon 1 J5 has a 21.0MP 1-inch (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features Expeed 5A processor.
Nikon 1 J5's sensor provides 5MP more than Panasonic GF8'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.
One other difference between these two cameras that is worth mentioning is that Nikon 1 J5'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 moire occurring in certain scenes.
Below you can see the GF8 and J5 sensor size comparison.
As seen above, Panasonic GF8 has a 1.9x Larger sensor area than Nikon 1 J5. 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.