Gladiator Set Swaps
The two most important set swaps for gladiator are mighty set vitality for the bonus damage in vitality, and the 2 piece sc + 2 piece oc set swap for whirlwind.
The former is self explanatory, the higher damage your glad has, the more damage it can deal. You can combine that set with the level 55 Gabriel jewel set for an even lower cooldown on vitality, as well.
The sc + oc swap gives gladiator a huge amount of aoe damage, as they are able to use whirlwind every 10 seconds. It does sacrifice a lot of defense, so it is usually best to do it if a spectre or wizard has frozen the mobs surrounding you.