The reason Paladins work well as the offtank on Gruul is mana lasts longer than rage. The offtank role is one big hit, then nothing for a while. So you take that one big hit, get your mana back from being healed and unleash your threat.
Also mana pots do more for a Paladin than Rage pots do for a Warrior.
The first part of offtanking Gruul is your prefight preparation. You want to boost your spell damage as much as you can for this fight.
If you have trust that your main tank won't go down, you can even sacrifice Uncrushability for more spell damage. However, our Main Tank dies quite often on our attempts, so I go with a fully uncrushable kit for this fight.
The two piece bonus of your Dungeon 3 set and your Tier 4 set are really helpful, but not necessary.
For buffs, you want Unstable Flask of the Sorcerer, Blackened Basilisk and Superior Wizard Oil.
Basically, on Gruul, I open up with a Judgement of Righteousness. I have to be careful because if it crits, I can pull aggro from the MT as he is still moving Gruul into position.
Then I nail him with Judgement of the Crusader. In my tank gear with Judgement of the Crusader up, I have nearly 600 effective spell damage.
I pop the wings (this is to say I cast Avenging Wrath), and get my SoV stack started, and Consecrate. Then I throw my Avenger's Shield. Now it's just a matter of watching my threat, and the melee dps threat, and trying to squeeze in between the two.
I have to watch for Shatters to know when to pop Avenging Wrath again. Since it's a timed buffed, I don't want to waste one second of it flying through the air or being stoned.
You can catch a streak where you dodge or parry a couple of Hurtfuls in a row. This is a good thing! However, it can leave you gasping for mana. That's why you always bring a stack of the Ogril'la Mana pots. Don't wait until you are completely out of mana to use it. Use it when you are around 1000 mana left. You want to give the cooldown some time so you can use it again if needed.