Born in Carmarthen, Davies was an all-round sportsman, representing Welsh schools at cricket and played for Glamorgan County Cricket Club's second eleven. He then studied chemistry at the former University of Wales, Cardiff and later studied for his teaching qualifications at St Catherine's College, Oxford.

Gareth played for Cardiff RFC for 12 years, and represented Oxford in the 1977 Varsity Match. During this time he gained 21 Welsh caps, captaining them five times, and winning a Triple Crown, and played on the 1980 British Lions tour to South Africa.

After retirement from rugby Gareth became Assistant Director of CBI Wales, and then Head of Sport for BBC Wales from 1988. In 1994 he was appointed Chief Executive of Cardiff Athletic Club/Cardiff RFC. In 1999 he joined S4C as Commissioning Editor for Sports and Events. He was also chairman of the Sports Council for Wales, and Director of Welsh Affairs for the Royal Mail. In 2006 he joined the Welsh Development Agency; heading the Sydney, Australia office.

In January 2009, Davies was appointed as Dean of the Carnegie Faculty at Leeds Metropolitan University, following which, in September 2013 Gareth took on the role of CEO of Newport Gwent Dragons. In July 2014 Gareth's name was put forward to succeed Gerald Davies on the Board of the Welsh Rugby Union, and was subsequently elected Chairman of the WRU in October 2014.