In the books
In the A Song of Ice and Fire novels, Gendel was joint King-Beyond-the-Wall with his twin brother Gorne, three thousand years before the War of the Five Kings. The two gathered the Free Folk and led them through a cave path beneath the Wall that would later be called "Gorne's Way". Gendel and Gorne's host evaded the Night's Watch, but when they emerged, the King in the North was awaiting them and attacked them. Gorne slew the King, but the King's son took his crown and slew Gorne. While the battle raged, the Night's Watch emerged from their castles and took the Free Folk in the rear.
According to northern tradition, Gendel was also killed in battle, as he was surrounded by all sides: Starks to the south, Umbers to the east and the Watch to the north. However, the Free Folk believe that Gendel survived the battle and, cutting his way through the ranks of the Night's Watch tried to return beyond the Wall through they same route the wildlings had come through. However, Gendel didn't know the way as well as Gorne and he and his followers got lost trying to find his way out. The Free Folk believe that during the night the sobs of Gendel's children can still be heard and that those who venture into the caves and go too deep will be devoured by them.