Why did Gandalf have a ring?

[Boromir88]

So Círdan just kept it without ever using it, until Gandalf showed up.~Angry Hill Troll

I wouldn't say Cirdan never used it. Cirdan did his fair share of leading armies in the wars against Morgoth, and then later on helping the Dunedain against Angmar. Now, Cirdan did reach a point where he decided to remain in the Grey Havens and stay there. But, he was very active in the previous wars, so I doubt he never used his ring.