Panasonic GH2 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine FHD processor. On the other hand, Sony a1 has a 50.0MP Full frame (35.9 x 24 mm ) sized BSI-CMOS sensor and features Dual Bionz XR processor.
Sony a1's sensor provides 34MP more than Panasonic GH2'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 GH2 and a1 sensor size comparison.
As seen above, Sony a1 has a 3.8x Larger sensor area than Panasonic GH2. 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.