The class skills are too important, in some cases, to give up, yeah.

Gladiators have it a bit easier than most; their class skills as Paladins are NOT immediatly relevant or useful until Shield Oath, so you could just go to dungeons as a Gladiator using more cooldowns from other classes. In THIS SPECIFIC CASE ONLY, you can make up for not being a job by being a class and using several cooldowns from classes you normally could not. Pugilist and Lancer, especially.

Still, it's not recommended unless you know your stuff, and in some cases it's downright a bad idea. Marauders immediately get their tanking stance from going Warrior, for instance.

Also at 50 your relic is job-specific and can't be used by your class.