Gunslingers's CC immunity requires them to sit in one place and don't move or they won't take advantage of the full 20 second duration.
Unremitting/Unstoppable lasts a grand total of 4 seconds. As Guardians don't have a baseline hard stun during which they can beat on you unopposed you can't really call a short CC immunity OP. Guardian tanks do have hard stuns of course but their damage is so pitiful that it's negligible.
In short, this is one of the stupider complaints.
First, you use "stupider" when inditing something else as stupid, really nothing more need be said on that front.
Second, in a game where you can kill someone in 4 seconds or dam close to it, yes it is op.
Third, QQing a lack of a hard stun has nothing to do with the subject as lack of a hard stun does not make a Guardian defenseless by a long shot.