Gareth is best known for the part of Roj Blake in the dystopian science fiction television series Blake's 7, but has taken roles in many other films and television programmes, including Adam Brake in Children of the Stones.
Thomas trained at RADA and is now an Associate Member. He has been twice nominated for a BAFTA for his performances in Stocker's Copper (BBC Play for Today) (1972) and Morgan's Boy (1984).
Some of his other television appearances include The Avengers, Coronation Street, Z-Cars, Special Branch, Sutherland's Law, Public Eye, Star Maidens, Who Pays the Ferryman?, Bergerac, By the Sword Divided, The Citadel, Knights of God, Boon, London's Burning, Casualty, Taggart, Heartbeat, Sherlock Holmes, How Green Was My Valley, Torchwood and Midsomer Murders.
In 2001 he appeared in Storm Warning, an audio drama based on Doctor Who by Big Finish Productions. He also played the part of Kalendorf in the Big Finish Productions Dalek Empire series.
In 2006 he appeared as a guest star in the Doctor Who spin-off series Torchwood, in the episode '"Ghost Machine".