Her soul was cursed to live on top of the hill for killing herself. Gadeneri's soul can be freed if her skull is returned to her remains. Her skull can be found in Knifepoint Hollow.
It is a crime in the Shivering Isles to commit suicide, the punishment for that poor soul is entirety trap on the Hill of Suicides. The only way to break the curse is if somebody is to return the skull to its ghost.