Gendel as depicted in Game of Thrones: Histories & Lore.
Three thousand years before the War of the Five Kings, Gendel and his brother Gorne ruled as King-Beyond-the-Wall. They gathered the free folk to them and marched them through Gorne's Way, a path through caves beneath the Wall. They evaded the Night's Watch, but when they emerged on the far side, the King in the North was waiting for them and attacked them. Gorne slew the King, but his son took up his crown and killed Gorne in turn. While the battle raged, the Night's Watch, made aware of the battle, sortied from their castles and took the wildlings in the rear. Gendel was also killed in the battle according to the traditions in the North.
According to the wildlings however, Gendel survived. He was now the sole King-Beyond-the-Wall and cutting his way through the Night's Watch he attempted to return beyond the Wall via the same route the wildlings had come. However, Gendel did not know the way as well as Gorne and supposedly got lost forever trying to find his way out.
Wildling legend says that the descendants of Gendel's people still dwell in those caves, attacking anyone who tries to find Gorne's Way and eating them.