Long ago, when trolls used to occupy the land of Tanaris, they built up Zul'Farrak, the sand troll city in Tanaris, with a sacred pool. From that pool the trolls summoned the huge hydra Gahz'rilla, so fierce that even his scales crackle with energy. The trolls worshipped him as a deity, but the summoning of Gahz'rilla is a well-kept secret of the trolls. Gahz'rilla is said to be a demigod.[1][2]
Large amounts of ornaments have been modeled after the beast Gahz'rilla. These ornaments are called "Gahz'ridian", after the hydra demigod.
The subject of this article or section was part of the Elemental Unrest, a world event that heralded the beginning of the earth-shattering Cataclysm. Once the world event has run its course, this will no longer be available.
With the Mallet of Zul'Farrak, Gahz'rilla's gong can then be rung. However, it is wise to clear the area first, as Gahz'rilla can knock back players.
As of patch 3.0.8, Gahz'rilla no longer needs to be summoned. He is simply present at the location he used to appear upon being summoned. The Mallet of Zul'farrak has since been turned into a rareweapon with fairly decent stats and rare skin.
All mobs around the pool should be cleared first, as Gahz'rilla will occasionally launch players through the air which may cause them to land in a group of mobs. Gahz'rilla does not automatically aggro to the players when summoned, and knockback damage can be avoided by landing in the pool of water where he is summoned.
Trivia
If the name is closely examined it sounds like "Godzilla". It also bears a close resemblance to Godzilla's opponent Ghidorah.