Its a reference voltage for signaling so (in my limited understanding) it is not additive to vcore. There's a detailed explanation of it at the techrepository here: http://www.thetechrepository.com/showthread.php?t=87
Raja's overclocking guide for the DFI P35 at Anandtech (sorry, too lazy to look up a link) also has a lot of good discussion about the GTL settings. Unfortunately I don't know how the DFI settings guidelines apply to the 790i bios which is different.
Basically all I did on the 790 was find max fsb with GTL on auto, then start adding 10 mv at a time. I was eventually able to squeeze another 10-20 stable fsb out of a q9450 doing this. Its a very touchy setting -- the difference between being OCCT stable and not posting can be as small as 5-10 mv.
Good luck!
EDIT: Here's some great specific info on calculating GTL settings for the 790i: http://www.anandtech.com/mb/showdoc.aspx?i=3283&p=18
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