DxO Labs has a website, DxOMark, that can be used to make various comparisons of cameras, lenses and sensors. To compare the various G series sensors, go to the menu on the left hand side, click "Camera Sensor" and then "Compare Sensors". Up to three sensors can be compared.
I first chose to compare the G12 and G11 sensors. These are essentially the same so I then compared the G12, G10 and G9. (I've linked my comparisons but the link doesn't seem to work for everyone.) The G12 has an overall score of 47, the G10 scores 37 and the G9 35 but it is more useful and interesting to examine the charts that compare the ISO sensitivity, signal to noise ratio, dynamic range, tonal range and color sensitivity. The charts are accessed by clicking on the rectangles just above the pictures of the cameras.
On the ISO Sensitivity chart, notice that the measured ISO of the G12 and G10 are relativity close to Canon's rating but the measured ISO of the G9 is noticeably higher than Canon's rating. For example, when the G9 is set for ISO 800, the real ISO is 1068. Taking another example, when making noise comparisons between the G9 and G12, if the G9 is set at ISO 1600, it is really working at 2281; the G12 set at ISO 1600 is really working at ISO 1378. To account for these differences, the remainder of the DxO charts are based on measured ISO.
To my way of viewing the charts, the G12 sensor turns out better in every way than the G9 and G10. The charts do not show image resolution because resolution would be a function of sensor and lens combined.