I believe it's for Glycemic Index, which is an indication of how food is absorbed by the body and its impact on insulin levels. There's probably a ton of stuff on these boards about it, but you can also checkout:
http://www.glycemicindex.com/
I believe there is also a GI diet of some sort, but don't know any details.
Hope it helps,
-Tia
PS. Someone correct me if I got it wrong! :-)