Based on the following review: https://www.ninjette.org/forums/showthread.php?t=9780
I recently bought a set of Dunlop GT501G tires for my GS500 (from BikeBandit.com). These replaced the tried-and-true Michelin Pilot Activ. It had seemed that tire choices for the GS Twin were becoming more and more limited, with Michelin discontinuing the GS sizes. Fortunately, the Dunlop GT501G should be here to stay for a while, as it is the stock tire for the Ninja 250 and 500. After having blasted around all my favorite twisties, I find the the GT501G tires to be very good. Their traction in a lean seems as good if not better than the Michelins.
Michelin makes the Pilot Street Radials now in the right sizes for the GS. I haven't had a chance to try them out, but they will be my next tire for the GS.