It's an industry trade show where teh various aspects fo the industry get together... a chance for retailers, publishers and distributors to get together, see what they are doing, and so on.
And it is called the GAMA Trade Show as it is a Trade Show put on by GAMA (GAme Manufacturers Association).
That's about all I know since it is too far away for me to go to.
Although last year d20 companies were gambled away in vegas at the GTS, all on the toss of a d6, if you believe the reports that came back from the 2002 GTS.