Brand New Gladiator here, tips....go!

[Alistaire]

Honestly the biggest piece of non-obvious advice is that flash enmity scales with your weapon damage. Tanks are the most gear-dependent of the roles; the better you're geared the easier your healer will have it, freeing them to DPS more if you're as geared as you can be. So more than anyone else, keep your gear up to date, especially your weapon and especially when you'll be on the lower end of the level curve for a dungeon.

[TheodoreMcIntyre]

Honestly the biggest piece of non-obvious advice is that flash enmity scales with your weapon damage. Tanks are the most gear-dependent of the roles; the better you're geared the easier your healer will have it, freeing them to DPS more if you're as geared as you can be. So more than anyone else, keep your gear up to date, especially your weapon and especially when you'll be on the lower end of the level curve for a dungeon.

Another less-obvious tip, damage mitigation buffs are multiplicative, not additive.
So if you have Shield Oath on (20%), and use Rampart (20%), you're not mitigating 40% of the damage, you're mitigating around 36%. So if you're going to pop a bunch of defense buffs, try to use buffs with effects that don't directly stack, like Rampart + Convalescence + Bulwark, or Sentinel + Foresight + Awareness. This will let you get the most out of each individual buff. If you really need to mitigate as much damage as possible, just keep in mind that when you're using Shield Oath (20%) + Rampart (20%) + Sentinel (40%), you're not mitigating 80%, you're mitigating 61.6%. Here's the math:

100 - (100 * .2) = 80 - Shield Oath
80 - (80 * .2) = 64 - Rampart
64 - (64 * .4) = 38.4 - Sentinel
100 - 38.4 = 61.6

The general formula is x - (x * y) = z, where as x = the incoming damage, y = the percentage mitigation (per buff) as a decimal, z = the resulting amount of damage you're actually taking.
So you plug in 100 for x, 20 for y (Shield Oath), and the result, z, is 80.
Then you plug in 80 for x, 20 for y (Rampart), and the result, z, is 64.
Then you just keep repeating for as many buffs as you need to math out