Am I a bad Tank?

[Endemerrin]

As mentioned in a post above, timing is a big thing with tanking. Half of your abilities as a Gladiator (Well, sentinel abilities) are fairly pointless when it comes to dungeons. Actual blocks only occur on physical attacks, so your Aegis Boon, Deflection, etc won't do any good against TP moves - which are really the main threat during the boss fights. Same goes for ranged attacks and magic attacks.

Sentinel and Rampart II are really your bread and butter as far as defensive buffs go,(Outside of obsess/defender, which I'd consider to be passive buffs due to their length) and the latter has an incredibly long recast rate. While you can never completely predict when a boss will use a TP move, it's fairly easy to "sync in" with the rate at which they use them - and you can plan your buffs accordingly.

A lot of it can be positioning, too. On Batraal, for example. It's possible to dodge every single one of his TP moves by being behind him before he's able to execute it. You can accomplish this by "dancing" back and forth between his legs, as he has a rather slow auto-attack. I can go through all of phase one without needing a single heal from a healer that way. You can also dodge the laser he spams in phase 3, so being mobile really eases up on the stress on your healers. Though not the perfect example since it was kind of a messy clear and I tend to grow less concerned with dodging everything in phase 2 and 3, there's some examples in here:http://www.youtube.com/watch?v=QJU27GaaG8g

Sitting in place like a rock trying to soak up all the damage works, but chances are your healers will have to pick their game up to compensate.

Having Second Wind II and cures available are also very important. It isn't worth it to blow them if you're sitting at a decent amount of HP, or if you aren't in a healing intensive phase.