Gehn cut down the Great Tree to create paper for linking books, after Atrus trapped him on Riven. Over time, the instability of Riven caused the islet to drift far away. The Moiety had no information about what was on the island.
Gehn initially used the structure as his private office.
Gehn trapped Atrus's wife Catherine in a prison he built on the top of the Great Tree's stump. Nelah was allowed to meet her. Gehn could spy on Catherine from his Survey room on the Survey Island.