Here's the basic of my experience using a Gladiator in Phase 3 (up to level 35), in a party situation.
Usage of Markers
The basic applies to any party here. You want your teammates to know what monster you should attack first. Its mostly a tool to try to point your teammates in the right direction. Without them, they tend to attack anything, and it makes the next party harder.
Enmity 101
Of course being a tank, the most important thing is keeping hate. The Gladiator, unlike Marauder, has one AOE tool for that, and then its single attacks. So it works a little differently, considering Marauder can use TP to keep Enmity on several target at the same time, whereas Gladiator is limited to the MP pool.
The first tool here is Flash. AOE, roughly 10 yalms, not a big distance but gets the attention of anything around. It doesn't decay too fast, but since you're not attacking them constantly, Healer will eventually pull hate. In my experience, you can reapply a dose of Flash everytime one monster dies and it seems to work. If you're worried, use it every 50% HP loss on the current monster.
Some guides tell you that you should rotate targets, but I found this to be a potential hazard personally. Its better to concentrate one monster down and just apply Flash regularly. The problem is, your MP here.
Normally your best enmity tool after Flash is constant use of Fast Blade -> Savage Blade (-> Rage of Halone later on). But if you use alot of MP you might want to use Fast Blade -> Riot Blade will give you a quick shot of MP. But you don't want to use it too often if Hate is wobbly.
Enmity 102
Your biggest issues when tanking as a Gladiator is because your Enmity is concentrating on a single target, it makes it impossible to keep hate if your teammates don't attack the same target as you do. Which is why you need markers, but if your teammate are stubborn or uncooperative, you might want to be the one to change targets.
The worst DD is the one that doesn't get the clue when they are not on the right target despite this. Normally, if a DD gets hate on a monster, they should stop attacking, let the hate rebalance itself. Most DD prefer just going the whole mile and instead it puts a strain on the healer. Not encouraged of course. Even worse, some will start kiting the monster. Your chances of getting hate just went below Zero if you can't put Flash on it. Don't blame yourself for poor teammates, but at least try telling them first.
Enmity 103
If you lose hate while they are on the same target as you, the reason is probably because they are vastly out-damaging you. This will happen if the RNG favors your DDs more than you for chests, especially for the weapons. Your DD needs to slow down, especially if you're already going full throttle and its not working. Don't feel responsible in this case, you're doing your best, your DD might need to be told to slow it a bit.
Sometimes its beyond your control though, and a monster will break off and attack the healer. Your DD should be tyring to help, get the hate off their healer, and pull it back to you so you can re-Flash it. But don't expect every DD to realize this either, don't count on it.
Enmity 104
Provoke is a hate tool that works differently. Its not a 'enmity boost' like Flash, Fast Blade, Savage Blade and so on. Its actually an ability that automatically sets your own enmity at 'highest enmity in your party +1'. As you might guess, this is not a hate building tool. But if a monster breaks off from you, its a good aiblity to bring it back and then reFlash it as normal.
Its also a good tool when multi-tanking in larger groups. Provoke sets you on top of the list, so the other tank can break off and rest MP or do another task, depending on what's needed.
Enmity 105
Shield Lob is not a bad enmity tool, but its mostly for pulling. I personally don't think its a good ability to use during a fight because of the TP cost. For one Shield Lob, you could have used Fast Blade + Savage Blade. But it can help restablish hate on a target after a Provoke, for example, especially if the target is not right next to you. Provoke + Shield Lob while the monster is far from you, and Flash once its close to you. The Shield Lob will give a enmity boost that will keep the monster from changing directions again, especially if a mistimed Cure happens that moment. The +1 Provoke gave you won't be enough to compensate for a Cure, but Shield Lob would give you a cushion until the monster is back in Flash range.