Well, I'm quite certain that simply following up on his name you'd get a general idea of why this guy is tough, the whole dragonslaying part and all.  So, not only does he massacre dragons with apparent ease, he also happens to be general overlord of every single last ogre in Outland to boot, making him a tough one to deliver the beat down to.  Regardless, read and learn.

Abilities

Growth - Self-buff, increasing damage dealt by 15% every 30 seconds and size is increased by 10%, can stack a total of 30 times.

Hurtful Strike - Occasionally attacks the 2nd highest player on the aggro chart within melee range, this requires a second tank to deal with as it deals around 12350-13650 damage.

Revereberation - Zone-wide silence lasting 4 seconds, can be reduced by anti-silence abilities.

Cave in - Randomly utilized AoE that causes 3000 damage every 3 seconds within the targeted area, this attack is preceded by the shaking of the ground around the area as well as visible upon the roof of the cave.  These will occur more frequently as the fight continues.

Ground Slam - Zone-wide knockback attack, followed up by Gronn Lord's Grasp.

Gronn Lord's Grasp - Stacking debuff that initiates after a ground slam, reduces movement speed by 20% and stacks 5 times before causing the Stoned effect.  This can only be avoided/dispelled by abilities of complete immunity such as divine shield as well as cloak of shadows.

Stoned - Stunned.

Shatter - Once your raid has been stoned Gruul will proceed to shatter everyone, the damage dealt is increased if players are within range of one-another, this deals physical damage and penetrates immunity such as divine shield and ice block.  Can deal everywhere from 1.5 - 9k depending on distance between players.

Fight

The fight will be about a maximum of ten minutes in length, beyond this point the stacking buff Growth will result in overwhelming damage being dealt to your tank and imminent death.  Additionally once you start bordering on 15 or more growth’s Hurtful Strike can start entering one-shot range for your off-tank.

For this fight high health and maximum DPS is preferable, melee DPs are at a disadvantage duet to the frequent cave-ins however.  Other than that the primary focus is on keeping your tanks alive despite the massively draining aspects of the fight.

Should you have more than one tank then hopefully if your main-tank drops the off-tank can pick up primary aggro duties whilst your third tank begins to eat the Hurtful Strikes.  It’s all about endurance and intelligent usage of abilities such as innervate.

Basically, maintain a constant stream of damage, should you lose too many players at any time you’re practically guaranteed to wipe due to the time constraints of this battle as opposed to others.

While this is somewhat endurance based it’s also required to balance the two, keeping badly beaten tanks alive whilst conserving mana for the long run, beating on Gruul hard enough to drop him without extinguishing your mana supply immediately, etc.

Follow these guidelines and should your raid be well enough equipped and prepared Gruul should drop in a rather predictable fashion excluding random chance/luck victories.