Gamma wasn't a bad person, but she was a bad character.
Later revealing her name as Mary, Gamma was given the title by Alpha after saving the Whisperer leader. Her task was to talk to Aaron and take advantage of his kindness to find out information about Alexandria and Hilltop, which ultimately backfired as she began to side with him and began to lie to those above her.
By betraying the Whisperers, she managed to indirectly cause the death of Laura, a reformed Savior, and the remainder of the Highwaymen who were guarding Alexandria.
Though she did give her life to protect her baby nephew, she didn't really have a purpose for the show, and was greatly overshadowed by Dante, another Whisperer who infiltrated Alexandria.
One thing she managed to do which was interesting was tearing off Beta's mask, which revealed his identity of Half Moon.