Gatland has been forced into a rethink in his pack after losing experienced players to retirement, including stalwart lock Alun Wyn Jones and loose-forward Justin Tipuric.
He could unleash several new names in a warm-up game against England on August 5th, which is followed by a second clash against the same opponents on August 12th, and a fixture against South Africa a week later.
He has left out number eight Taulupe Faletau, along with Owen Williams, Joe Roberts, Johnny Williams and Alex Cuthbert, though they remain part of his World Cup plans, according to a media release from Welsh Rugby.
Thomas won seven caps for England in the 2013/14 season but qualifies for Wales through his father. He has been able to take advantage of the same World Rugby eligibility rule that allowed Jean Kleyn to play for South Africa this month having previously represented Ireland.
Gatland is clearly uncertain of his front-row options having also called on uncapped props Corey Domachowski, Keiron Assiratti and Kemsley Mathias.
Versatile back row forward Teddy Williams and former New Zealand junior international flanker Taine Plumtree are other uncapped players in the squad.
There are also some famous names among the new backs. Centres Keiran Williams and Max Llewellyn have been included, the latter is the son of former Wales lock Gareth, who at one stage held the team record for the most international caps with 92.
Also named is fullback Cai Evans, son of another notable former Wales international, Ieuan.
Wales squad
Forwards: Corey Domachowski, Kemsley Mathias, Nicky Smith, Gareth Thomas, Elliot Dee, Ryan Elias, Dewi Lake, Sam Parry, Keiron Assiratti, Tomas Francis, Dillon Lewis, Henry Thomas, Adam Beard, Ben Carter, Rhys Davies, Dafydd Jenkins, Will Rowlands, Christ Tshiunza, Teddy Williams, Taine Basham, Dan Lydiate, Jac Morgan, Taine Plumtree, Tommy Reffell, Aaron Wainwright
Backs: Gareth Davies, Kieran Hardy, Tomos Williams, Gareth Anscombe, Dan Biggar, Sam Costelow, Mason Grady, Max Llewellyn, George North, Nick Tompkins, Keiran Williams, Josh Adams, Rio Dyer, Cai Evans, Leigh Halfpenny, Louis Rees-Zammit, Tom Rogers, Liam Williams.
