Here's what I know, somebody correct me if I'm wrong:
GT made two track bikes - the Pulse and the GTB. The Pulse was the nice one, and the GTB, well, that stood for "Generic Track Bike". Both frames were aluminum and looked quite similar, and to tell you the truth, I'm not sure what the difference was. Probably a nicer alloy on the Pulse. The Pulse was made for a longer time than the GTB, and older Pulses looked more traditional. Both were designed for the track, so they have a short wheelbase and tall-ish bottom bracket.
I have a GTB, and it's not the nicest looking frame in the world - the welds are big, globby, and asymmetrical. But, the frame is straight, and has proven to be durable on the streets of NYC and DC for almost ten years now. The frame sizes are just like any other bike, but when you set the bike up so it fits you, it will look weird because there will be very little seatpost showing.
The headset is 1 inch threaded... what else can I tell you? I paid something like 700 dollars for mine in 1996. Most of the parts that came on it were trash, except the cranks and bottom bracket. I have replaced every part on mine.
Oh, the aluminum rear fork tips are really soft, and are easily chewed up by the track nuts when you tighten the wheel.