Panasonic G100 has a
20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor . On the other hand, Panasonic G2 has a
12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine HD II processor.
Panasonic G100's sensor provides 8MP more than Panasonic G2'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 G100 and G2 sensor size comparison.
Panasonic G100 and Panasonic G2 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. On the other hand, since Panasonic G2 has 67% larger pixel area (18.74µm2 vs 11.16µm2) compared to Panasonic G100, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.