Due to bios updates and microcode updates its alot more difficult to undervolt now. You need to downgrade bios to 306 I, and theres a windows update that breaks software undervolting (you will have to look into which one it is and how to remove it). After that you should be able to undervolt again.
Since asus hasnt released a Bios for the g531GT that has overclocking capabilities you can only do it with software, (I have been looking into a modded bios but no news on that front as of yet, although there is a 'Virtual Bios' that you could try, the developer uses a g531gt as well. He used it to install mac on his g531gt. I never got it working correctly however from what I understand rather then loading the windows bootloader, you load the virtual bios, then the virtual bios loads windows with whatever settings you changed in the Virtual Bios)
Ive uploaded the bios somewhere else you might be able to find it if you look for it.