This is another fight that is fairly easy once you get the hang of it. Gruul will grow in size and strength throughout the fight. You want to kill him by his 11-13th grow or he will be hitting so hard most healers new to 25 mans will not be able to keep your tank alive. MD whenever your cool down is up on your MT and OT because the Shaman and Warlocks will be right on their tail on threat. Gruul has an ability called Hurtful Strike that will hit the second highest on the argo so you want 2 tanks up there.
You will need to watch out for Cave Ins. It will look like rocks are falling from the ceiling and they do about 3k damage every 3 seconds. If you see you are in one move asap. Your pet will also take damage from this.
Last big problem to deal with is the Shatter. Gruul will throw you somewhere and you then have 5 seconds to move at least 15 feet away from anyone else. Oh yeah, and you are slowed during that 5 seconds so you are walking. Then you freeze in place and if you are close to anyone else all in the close range will take damage. Many raids will say to stay so far apart from each other while fighting but then when you think you are doing really good with your neighbors someone drops from the sky and blows it all to hell. Don't worry about your pet during this time because they will not cause or take any damage.
Keep an eye on your pets health through the fight. Call it back when needed for heals, and stay under the MT and OTs threat.