The only questline that you'll have to start over are your class quests, since you'd be leveling a different class. So you don't have to worry about your main scenario quests or any non-class related side quests you've done, and since you've already got gladiator leveled a bit, some of its abilities might be available and useful to you cross-class. That and you can always go back and pick it up again from where you left off once you're more familiar with the game.