Thomas made many television appearances included The Avengers, Coronation Street, Z-Cars, Special Branch, Sutherland's Law, Public Eye, 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. He appeared on stage in many productions. His appearances include RSC productions of Twelfth Night, Othello and Anna Christie; English Shakespeare Company productions of Henry IV, Part 1 and Part 2 and Henry V; and King Lear, Educating Rita, Cat on a Hot Tin Roof, The Crucible, Equus and Déjà Vu. In 2010 Thomas gave an acclaimed performance as Ephraim Cabot in Desire Under the Elms at the New Vic Theatre. In 2000 Gareth was the first subject of MJTV's interview/drama CD series The Actor Speaks. He talked about his character Blake in Blake's 7 in one of the interviews. From 1999 to 2005, Thomas took part in 12 of 14 CD episodes of MJTV's original audio comedy sci-fi drama seriesSoldiers of Love, playing camp Welsh TV host Hywel Hammond and villain of the piece Aaron Arkenstein. 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". In 2012, Thomas returned to the role of "Roj Blake" in Big Finish Productions' Blake's 7: The Liberator Chronicles, a series of dramatic readings which take place during Series One before the death of Olag Gan. Thomas stars as "Blake" in Counterfeit by Peter Anghelides and False Positive by Eddie Robson. In 2013 he appeared as Brother Cadfael in Middle Ground Theatre Company's adaption of The Virgin in the Ice by Ellis Peters. He was twice nominated for a BAFTA, for his performances in Stocker's Copper and Morgan's Boy. Morgan's Boy was his favourite television role, but it was Blake's 7 that won the bigger audiences.
Thomas's first wife, Annie, was the mother of his two children; this marriage ended in divorce. His second wife, Sheelagh, was a make-up artist; this marriage also ended in divorce. His third wife was Linda. After many years living in the Scottish Borders he moved to live in the county of Surrey in 2009.
