The 25-year-old openside has 14 caps for Wales, and joins a side who also announced the arrival of Bath's Samoan wing Elvis Seveali'i, who helped keep the English club in the Zurich Premiership.
Thomas joins Bath scrum-half Gareth Cooper and back rower Nathan Thomas in heading back over the Severn Bridge, but with fellow Bath and Wales back rower Gareth Delve currently sidelined with a serious shoulder injury and American Dan Lyle set to go the World Cup, the Zurich Premiership side could well be short of cover in that area, although England's Michael Lipman has joined from Bristol.
Neath/Swansea coach Lyn Jones said after announcing the capture of Thomas: "Gavin is excited and positive about the challenge that is ahead of him individually, and the region as a whole.
"I am sure his tremendous natural enthusiasm will have a positive effect on the coaching staff and other players.
"He brings with him a level of speed, experience and skill level that would make him a valuable addition to any side, and we are delighted to have him on board," he said.