Canon G12 has a 10.0MP 1/1.7" (7.44 x 5.58 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Canon G7 X MII has a 20.0MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features DIGIC 7 processor. Canon G7 X MII's sensor provides 10MP more than Canon G12'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 G12 and G7 X MII sensor size comparison.
As seen above, Canon G7 X MII has a 2.8x Larger sensor area than Canon G12. 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.