I found it in a chinese forum (bbs.gfan.com/android-2014369-1-1.html)
I'm sure most people found the G button useless, so it would be nice to change that. The following is my experience with it:
The position of the G button make it perfectly useful as lock/unlock button, you can use it to shutdown as well, but cannot use it to switch on the phone, I guess it is electrically hardwired to the power button at the top.
Not too bad for camera, long press it will launch the camera app, but it don't have 2 levels so press it once and it will start auto-focus and take picture immediately, similar to touch and release shutter button in camera app, which mean you can't "dis-engage" a failed focus attempt.
here is the method:
1. make sure you have root and r/w to system partition
2. the file in question is "TWL4030_Keypad.kl", it's in /system/usr/keylayout/
3. before making any changes, copy the file to SDcard as a backup
4. open the file with any text editor, scroll down to 3rd line from the bottom, it should be "key 95 GESTURE"
5. if you want to use it as camera button, change it to "key 95 CAMERA WAKE_DROPPED"
if you want to use it as power/lock button, change it to "key 95 POWER WAKE_DROPPED"
6. save the changes, reboot to see the changes.