Two entries with the name Ganelon.
Ganelon
A baron who served King Mark of Cornwall.
He conspired with two compatriots, Godoine and Denoalen, to expose the affair between Tristan and Isolde.
Source
Tristan | Béroul, late 12th century
Ganelon
A traitorous servant of Charlemagne, whom he accompanied to Britain in the eighth or ninth century.
They visited the Tower of the Dead, which Arthur’s knights had erected on the plain of Salisbury, with Mordred’s head hanging in the wind. Ganelon, perceiving that the Tower was a warning to all traitors, had the head cut down and buried in an undisclosed location.
Source
Post-Vulgate Mort Artu | 1230-1240