haha, GI is an interesting one, in that it's been around for years although i have noticed a huge influx of mentioning it when advertisers sell products on TV. To me, it seems to be another health buzz word
albeit it is actually useful to know about.
Anyway, it's all about the rate in which energy is released and the rate in which the carbohydrate is broken down in your system/blood stream. Low GI foods release energy more slowly and hence give you a sustained amiount of energy, instead of a quick buzz and also make you feel "fuller" for longer.
as for actual quantities and presices measurement, i don't really know much about that. however wholegrain breads etc where you can actually see the grains etc are low GI. The more the food is process and refined, it seems, the higher the GI as they're removing natural elements of the carbohydrate that woul naturally be broken down in your sysetm, i guess?
ok, i found this site, which will probably be better than me drivelling off of the top of my head
http://www.glycemicindex.com/