Gamma was a member of the Dark Ancient organisation, but was not in fact a Dark Ancient. Rather, he had a similar but completely separate method of prolonging his own life by forming a collective of multiple stolen Warforged bodies. By the time of the campaign, he had accumulated so many bodies that he existed as an almost amorphous pile of bodies and as such could not travel. He was "fed" by the other Dark Ancients, who brought him Warforged to feed off.
Unlike the other Dark Ancients, who "ascended" due to their extreme intelligence and mastery of psionics, Gamma gained the ability to take over other bodies through an almost infantile fear of being alone. This lack of intelligence (at least compared to the other Dark Ancients) may be the cause of his inability to speak.
His part in the Dark Ancient masterplan was to guard Polaron until Delta could come to assist Epsilon in seizing control of the facility. However, he was insufficient to prevent the party from entering, and they killed him. As he could not mind-jump to a new body, he became the second Dark Ancient to fall to the party.