Gehn's parents were
Aitrus and
Ti'ana. Because he was the last of the
D'ni (until son
Atrus was born), Gehn was not properly trained in the art of
Writing. Because of this Gehn developed the
hubris which all D'ni were taught to avoid: he believed he was actually
creating the worlds he wrote, and thus considered himself a
god. In fact, writing the special descriptive
Books does not create worlds; it merely creates a link to a world that
already exists with the given properties.
Riven was always unstable because of Gehn's
lack of understanding of how to properly describe worlds, and his changes to the descriptive book teneded to do
more harm than good.
Gehn became trapped on Riven when Atrus used the last Myst linking book at the same time he threw it into the Star Fissure, where it was presumed lost forever.