Dunlop GT501: The Racers' Choice,
A quick diatribe on why GT501's are the racers choice.
The GT501 isn't a race compound, it's a standard sport touring compound. The main reasons it was chosen over other tires are:
1) Dunlop always has a great tire vendor, Sport Tire, at the track on race weekends.
Finally, their vendor, Sport Tire, pledged to have the tire readily available and was willing to directly petition the company to hurry a production run when stocks grew thin.
GT501's are not the holy grail of Ninja 250 racing, though. Those with more $$ than brains, or those racers who have exceeded the capabilities of the GT501 (rare) often opt to have Kosman "weld-up" their wheels to provide access to a wider range of tires, including some high performance radials. This can cost upwards of $1700 though, and the tires are commensurately more expensive.