Gethin attended Radyr Comprehensive School, Cardiff and played for Wales U16 and U21.
He joined Newport at age 17 playing for the Youth and Under21 sides. He was loaned to Pontypool in his last year as Under21 and played 5 games helping them win Division 1 in 2002-03. Made 1 appearance for Newport RFC in 2002-03 with his debut against Cardiff in April and 31 appearances scoring 4 tries in 2003-04. Also played as replacement for Newport Gwent Dragons. Development contract with regional Dragons side 2004-05. Nephew of Alun Billinghurst, former Newport scrum half.
Made 100th appearance in 2006-07 and his 150th appearance on 18th April 2009 away to Llandovery (Won 16 v 32).
Final season for Newport RFC was 2013-14.