Gesler was originally a captain under the Old Guard, then a sergeant, then a corporal in the Malazan Coastal Guard, later promoted to back to sergeant in the Bonehunters. He was one of the last of the Boar Cult. He and Stormy had been friends since they were young.
Gesler was hired by Duiker during the Seven Cities rebellion (the Whirlwind) to pick up a fellow historian, Heboric Light Touch, from the Otataral Desert on a nearby island. Accompanying him were Stormy, Truth, and the mage Kulp. After rescuing the historian, he travelled briefly through a warren where he, Stormy, and Truth somehow began the process of becoming Ascendants.
Gesler and his tent-mate Stormy were abducted from the Bonehunter army camp by the K'Chain Che'Malle and Gesler was installed as their Mortal Sword. With the help of Stormy as well as Sinn and Grub they defeat the K'Chain Nah'ruk.
Gesler and Stormy lead the K'Chain Che'Malle to join up with the Bonehunters.