This says it will fit a gs500. If it did I think it would be awesome!
http://cgi.ebay.com/ebaymotors/Suzuki-Katana-GS500-Bandit-Tail-Bag-Seat-Cover_W0QQcmdZViewItemQQhashZitem19badf4870QQitemZ110509377648QQptZMotorcyclesQ5fPartsQ5fAccessories
Mary
I suppose it will only wrap around the back and sides of the seat. It looks like it was designed for bikes with separate front and rear seats.