Jonathan Thomas, Andy Powell and Richie Rees are not travelling as expected and Ken Owens, Lou Reed, Andrew Bishop, John Yapp and Scott Andrews will also be left behind.
But Stephen Jones, who missed the first training camp for the birth of his first child, is heading to Poland.
Gareth Delve and Dwayne Peel are unavailable while Richard Hibbard and Jason Tovey will remain in Wales because of injuries.
Gatland, who selected an initial 45-man training squad, has cut that down to 35 as he takes the players for a second week of intense training.
"The nature of the training regime next week meant we were always going to take a smaller group of players, because of the strong rugby element involved," he said.
"The players on the camp have an opportunity to work hard and impress, but it will not be a case of out of sight out of mind for those players who remain at home.
"The door is still very much open for those players who are not making this trip."
Jonathan Thomas' omission comes after the Ospreys forward played in all but one of the Six Nations matches last season, and played the full 80 minutes in the autumn internationals against Australia, South Africa and Fiji.
Rees, 28, was suspended for the whole Six Nations campaign, but the Cardiff Blues scrum-half started two games in the 2010 tournament.
Powell has not featured since starting against England in the 19-26 defeat to England in last season's Six Nations opener.
Wales are carrying a number of injured players in the training squad, with Leigh Halfpenny's ankle injury of particular concern for Gatland.
But prop Gethin Jenkins, who missed the entire 2011 Six Nations campaign after having foot surgery, is approaching full fitness and hoping to return for the three warm-up games against England (twice) and Argentina in August.
After the three warm-up Tests, Gatland must name his 30-man squad for September's World Cup in New Zealand by 22 August.
Wales squad travelling to Poland:
Gethin Jenkins (Blues), Lloyd Burns (Dragons), Huw Bennett, Ryan Bevington, Adam Jones, Paul James (Ospreys), Matthew Rees (Scarlets), Craig Mitchell (Exeter Chiefs). Bradley Davies, Sam Warburton, Martyn Williams (Blues), Luke Charteris, Danny Lydiate, Toby Faletau (Dragons), Ryan Jones, Alun Wyn Jones, Justin Tipuric (Ospreys), Josh Turnbull, Rob McCusker (Scarlets). Michael Phillips (Bayonne), Lloyd Williams (Blues), Tavis Knoyle (Scarlets). Jamie Roberts (Blues), James Hook (Perpignan), Jonathan Davies, Stephen Jones, Rhys Priestland, Scott Williams (Scarlets) Gavin Henson (unattached). Leigh Halfpenny (Blues), Lee Byrne (ASM Clermont Auvergne), Aled Brew (Dragons), Shane Williams (Ospreys), George North, Morgan Stoddart (Scarlets).
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