Guess you missed where a lot of a tanks survival comes from gear. If they're running DPS gear they don't have much defense, shield or absorb. They have what little survivability they get from skill trees.
They can still outdamage a gunslinger, partly due to the fact that they can stay alive longer in the first place to continue dishing out damage. A hybrid implies that your spec will be mediocre at both dealing damage and tanking, but it seems that some classes are good at both.