Schoolfriend of Noko and Howard Gray, with whom he later formed the band Apollo 440, Gardner spent much of the 1980s in London playing and programming keyboards and synthesizers for a variety of artists. During this time he also formed the short-lived band The Umbrella (which included Noko), where the lifelong John Barry fixation can be seen. Later, Gardner was involved with Luxuria, the band formed by Noko and Howard Devoto. In 1990, he co-founded Apollo 440, a band and remixing team.
In 1993, Gardner left Apollo 440 in order to concentrate on composition, which had always been a parallel interest, and he moved to New Zealand in 1994, becoming a citizen of the country in 2001. In 1996, he set up the contemporary music ensemble 175 East, of which he is currently director and occasional conductor, and his compositions have been played and broadcast throughout the world.
Gardner is also a broadcaster on music on the New Zealand radio station Radio New Zealand Concert. He writes and presents shows covering contemporary music with shows on characters as diverse as John Cage, Frank Zappa and the James Bond soundtracks of John Barry.