The GI diet is about getting cutting back on "bad" carbs.
This website gives good info: www.glycemicindex.com
A high GI food raises your blood sugar very quickly, which in turn causes an insulin surge. Excess blood sugar ends up being stored as FAT.
Foods that have a low GI don't raise your blood sugar so fast, keep you fuller longer, and generally tend to be healthy, whole foods.
If you eat veggies, whole grains, lean meats, fruits, and some dairy (and eat as clean as possible) you are basically eating a low GI diet.