You can find the Gundrak dungeon within Zul’Drak.
The coordinates are (76.4,21.8)
As you kill bosses you need to click on the Altars behind them (shown as * on map). Once you activate all these items it will turn the middle platform so you can reach final boss.
You can only access the room to 4th boss Eck the Ferocious on Heroic mode. The small door will open up after killing 3rd boss Moorabi, it will take you underground.
2. Drakkari Colossus
/i Inflicts physical damage to a player. At 50% health releases Drakkari Elementals from his body. Elementals inflict nature damage to players. When Drakkari Elementals at 50% health Merge rejoins them & the boss.
4. Eck the Ferocious (Heroic Boss)