Commenting on the signing Rugby Manager Robert Beale said: "This is a fantastic opportunity for us to be able to sign Gavin. He brings tremendous experience to our back row and is a ready-made replacement for Richard Parkes.
"Following his injury last season, Gavin is keen to get his career back on track and I am confident he will have a major impact in the team this season. Once again this signing demonstrates our commitment to the blending of youth and experience."
Thomas said, "I am delighted to be joining the Dragons after being out for most of last season. It's great that the Dragons have shown confidence in me and I am looking forward to repaying them by performing on the pitch this season. There are already some excellent players in the back row and I aim to battle my way into gaining a starting place in the side and contributing to the overall development and success of the team."
"I have known Gavin for sometime and am delighted to have secured his services for the team," said head coach Paul Turner. "He has a proven track record and with 22 Welsh caps has wealth of experience which he can bring to the back row and adds more strength to our squad."