From the Burning Crusade Bestiary:
Monstrous. Cyclopean. Terror incarnate. Words cannot begin to describe the terrible gronn of Outland, the immortal demigods of the ogre race. Some say the gronn gave rise to the lesser ogres, yet if so, they show but little love for their children, as the savage brutes lord over the ogre clans with an iron fist. There are said to be only seven gronn in all of existence, and they are not famed for their intelligence. Nonetheless, such rumors are cold comfort in light of the undeniable fact that the gronn wield devastating power.
From the official WoW Burning Crusade Strategy Guide:
Two of the known Gronn are in Nagrand. One of these is powerful but is still likely to be felled by a smart assault for two or three heroes. The other nicknamed The Hungerer, is an amazingly potent foe; many think it would take five of the finest champions of Nagrand to stop such a monstrosity.
Farther north, in the Blade's Edge Mountains, more Gronn have been seen. Legends tell of a lair in that area where the greatest of these Gronn may be located.
- Gruul the Dragonkiller, the raid boss in Gruul's Lair is the father of the Gronn.
- Given that it is said there are only 7 Gronn known to exist, two remain after Gruul's sons; Nath is speculated to be the final Gronn.
- There are a few elite monsters in Nagrand, called Mountain Gronns, that are walking around near Warmaul Hill and will fight with Ogres. These are probably lesser Gronns, or a step between Ogre Lords and Gronn.
- At The Circle of Blood in Blade's Edge Mountains there is a NPC, Baron Sablemane that has this to say: "My loathing of interruptions is overshadowed only by my hatred of Gruul the Dragonkiller and his seven sons!" This complicates matters further, implying that the 7 are the sons of Gruul, and that any other "Gronn" are not counted among the seven. At the very least, this seems to invalidate the "only seven in existence" statement.
Given that both Durn and Maggoc are described as Gruul's sons, Gruul may be the first Gronn, or else the being who created them (if this is the case, then Gruul might not count towards the total of seven). Given the use of the term "demigod," it's logical to assume that Gronn are comparable in power to the Azerothian variety, such as Malorne, Cenarius, or Aviana.
But, among the demigods of Azeroth, there are huge differences in power. There are the mightiest demigods, such as Malorne, Cenarius, Aviana, Agamaggan, Ursol and Ursoc, but then there are the lesser ones, such as the little guy with the sickle, the six-legged wolverine, the tiny fox among others.
It is most probably so, that the seven gronn are demigods, but that there are differences in power amongst them, just as with their Azerothian counterparts. Gruul the Dragonkiller is the mightiest one; as mighty as Cenarius, Malorne or Aviana, albeit in a different way, while the other gronn, Durn, Maggoc, Nath, and the patrolling ones are lesser demigods, akin to the lesser Azerothian demigods.