Story quests that offer gear usually offer you a choice of different pieces. A typical pattern is for a quest to offer three pieces of the same type (all helms, for instance, or all leggings), but one is for tanks, one for non-tank DoW classes, and one for DoM classes. I expect people most often choose the one relevant to the class they're on at the time, but if I already have that piece, or I'm already over-level for it, I'll sometimes pick up another that will be useful when leveling another class later. It's up to you which you choose.

As for gladiator-only gear, it's more common than gear specific to other classes. Tanks need a lot of vitality, so there's gear for them which other classes aren't allowed to use (since that much Vit would be over-powered on a non-tank class). On the other hand, gear that's designed for magic classes is generally allowed for "all classes" but just isn't useful to any other than casters. (It's the ones whose stat boosts are on Int and Mnd, which are essential to DoM classes, but you don't get much benefit from as a GLD.) So, while each class type has roughly the same amount of gear designed to be useful, tanks get more that's exclusive to them by the rules of what's allowed.