From my personal experience both playing the class and being in a team with the class at low level dungeons, Gladiators seem to be deficient in tanking capabilities.

I believe the fact Flash doesn't cause damage is pretty much the main reason Gladiator can't tank well enough. Marauder also causes more damage in the long run, I made some calculations before, in 30s, at lv.26, Marauder causes 3948 worth of potency damage, while Gladiator causes 3367. In 20s Marauder gets 2928 while Gladiator gets 2041. The reason why Gladiator picks up later on is because Fight Or Flight lasts longer than Berserk, 20s to 30s. But the start of Gladiator is slow compared to Marauder, and this damage I calculated doesn't even consider the Overpower/Flash start, just the main combos with available damage buffs of the classes. Gladiator stays too much behind in enmity production.

My personal suggestion is to level up Gladiator outside dungeons until you get Rage of Halone, because if you don't you will have a pretty frustrating experience.
If you still want to level up in dungeons anyway, mark your enemies' order of attack, don't double pull so that pesky Archer or Thaumaturge doesn't start AoEing, because if they do, bye bye aggro of half the mobs, because as a Gladiator you just can't handle causing enough enmity to many enemies at once like Marauder can, and pray everyone will behave and do as you tell.

My second personal suggestion is to drop Gladiator at Lv.22 and play Marauder, for Provoke. That's it.