Gruul is the first boss encounter in the Slag Works (1st wing) of BRF.

This fight is very similar to the BC encounter vice a few new abilities.  Its not a 'phase' fight per se, but it does go back and forth between 2 'phases' until dead.

For a more detailed view of his abilities or loot, please view the in-game dungeon journal or click HERE to view at WoWHEAD.

Abilities

Inferno Slice - Frontal slash attack that deals heavy damage on the current tank.  Stacking debuff that increases damage taken.  Also applies MASSIVE fire dmg split amongst players hit, giving a debuff that increases dmg taken from IS by 20% per stack and also leaves a light/moderate fire DoT on all hit.  Tanks need to stack with set group of players to split the fire damage.

Cave In - Ceiling collapses over players.  Deals light/moderate phys dmg every sec you stand in it.  Noted by circles of crumbling ceiling.. should be no more than 3 up at any given time.

Overwhelming Blows - Heavy phys dmg to enemies within 10 yards.  Stacks, increasing dmg taken from OB by 10% per stack.

Petrifying Slam - Same thing as BC encounter.  Knocks players in random direction, and starts petrifying players.  Once players receive 6(as per BETA) stacks of petrification they Shatter - dealing mild/moderate nature dmg to anyone with 8 yards.

Overhead Smash - Slams ground in a random direction, dealing heavy nature dmg.  Noted by line of crumbling ground when casting.

Destructive Rampage - Becomes blinded by rage, increasing haste by 50% and repeatedly slams the ground.  During this phase he will ignore all threat targets and face random directions and spam-cast Overhead Smash.

Note that the abilities listed here are from the LIVE Dungeon Journal.  If you watched the FATBOSS BETA vid, the fight has changed fairly drastically.  Cave In occurrs throughout the fight, there should be no more than 3 cavei-ins occuring at any given time, and the 2nd "phase" does not have large cave-ins to dodge any longer.  He simply deals heavy AoE dmg to the raid and spams Overhead Smash in random directions.

"Phase 1" - lasts approx 1 min 50 seconds

Tank Gruul in the middle-ish of the room, facing away from the raid.  Ideally the Off-tank should be directly behind the boss, and have the raid split into 2 groups, one on each side/foot of Gruul.  Main Tank will deal with Gruul for 60 seconds(ish) to deal with the tank debuffs, and pay attention to Gruuls Rage Bar and/or DBM timers.  When he nears 100 energy he will use Inferno Slice, the MT needs to run to 1 of the 2 stacked grounds to split the fire damage.  Alternate groups when doing this so one ranged group doesnt take 2 in a row.  This MIGHT be different than BETA, so you MIGHT have to have one group take 2 of the IS in a row, depending on the duration of the 20% debuff.  Pay attention to when he casts Overhead Smash and spread for it, then quickly regroup.  When he does Petrifying Slam, do your best to spread at least 8 yards so you dont Shatter on someone else.  STAY OUT OF FIRE (aka Cave In).

"Phase 2" - lasts approx 30 seconds

Gruul will move to the center of the room and go into Destructive Rampage, and spam Overhead Smash so be sure to move/spread when appropriate.

TLDR/Saedo's Cliffs Notes

Short version, 2 stacked groups, boss does Inferno Slice, fire damage that splits amog all it hits.  But also, Overwhelming Blows, a smash the tank and all around him damage attack.  All on the tanks to only move into a group during Inferno Slice, and away from a group during Overwhelming Blows.

Next up, Petrifying Slam, just like back in Burning Crusade, get knocked back, spread out and don't kill other people.

Then there's a phase where he just cause cave ins and raid wide damage.  And it repeats back and forth between above abilities and this.