Here's some pics, it's actually a ZX-11 ZZR1100 cap, but identical to the GTR and practically every Kawasaki since 1986.
The ring is held in place by the rubber seal. Removal should be by undoing the four screws that hold the two parts of the cap together and forms the lock and flap.
I suspect you need a new seal or have the ring mounted on the wrong side of the seal. Anyways hope the pictures help.