I've always assumed you'd need to remove the crystal ring and replace the gasket to achieve a watertight repair. Can you remove/replace the crystal without removing the crystal ring?
As long as you're careful not to crush the gasket when re-installing, it'll work fine.
On a gen Sub or SD I'd surely recommend a new one but on our reps it's not going to be a problem if you're careful. I've used the same gasket many times either replacing a crystal or re-centering a cyclops and never found it not to test OK to 5 ATM.