Do a little searching on this forum and you'll find out that it stands for Gran Tourismo Inezzione or Grand Trourisme Injection, whatever language you prefer.
It's also called a "Golf GTI" in Germany too, but on this side of the pond, where there is no more golf, it's a GTI. It was probably VW's strategy to release it before the new Rabbit, so that nobody calls it a Rabbit GTI.