|The following is based on Fallout Tactics and some details might contradict canon.|
Sometime after their departure, Gammorin's mutants were pursued by Brotherhood of Steel airships. While most of them ended up in the vicinity of Chicago, far from Gammorin's forces, one of the zeppelins crashed north of the mutant encampment. The leader, Paladin Latham, challenged Gammorin to a single unarmed combat and, to surprise of everyone, killed the mighty mutant.
As he defeated the previous chieftain, the super mutants expected him to become the new leader of their army. Latham suffered a head injury during the combat, and throughout the years, he grew more and more insane, even though he managed to organize the mutant forces into an army nearly as powerful as they had been during the Master's reign.
Gammorin is only mentioned in Fallout Tactics: Brotherhood of Steel.