G'Hanir was the Mother Tree and the domain of the demigoddess Aviana. It was not in the mortal world, but was tied to it in a fashion similar to the Emerald Dream. It was described as the tallest tree atop the tallest peak, with its fruit containing the seeds of all Azerothian trees. It served as an afterlife for all winged creatures of Azeroth, who could be seen flying amongst its many branches.
G'Hanir apparently died during the War of the Ancients when Aviana was (presumed or almost) killed by demons. ( Other sources claim Aviana survived through the War of the Ancients. ) ( Whether she was never slain or regenerated later is unknown, but it is unclear what this would mean for G'Hanir. )