We ('cafe.naver.com/op3d') were eager to find way to root for a month, but when we find it, it was too simple, actually.....
Just go back to the 'ROOTABLE latest version'(not 'just latest'!) of GB,
and root your device,
then update to latest GB, with the UPDATING APP LG PROVIDED(look at the image I uploaded. In Korean rom, it is called EASY UPDATE('OmaDmClient.apk' , in uploaded image file), but I don't know it's name in other nations' rom).
DO NOT update with PC. It's no use. Update with LG Updating app ONLY.
Reset your system partition before updating, or you might fail to update.
If your nation's LG phone doesn't provide Updating Application made by LG, then I can't give you any solution..... sorry.
Warning : I just recommend, not confident about it,
because I don't have any idea whether this method to root is available in other country.