Gareth's eagerness to have a "snog" with Adeola Oshodi led him to his death when he went off to find a secluded place to meet her. Because he took longer than Adeola intended, she followed him to her death.
Gareth was cybernetically altered by Cybermen from Pete's World, as well as Adeola, who also led Matt Crane to his death. He was killed when his earpiece was overloaded by a signal from the Tenth Doctor. (TV: Army of Ghosts)