Powell along with Jonathan Thomas, Richie Rees, Ken Owens, Lou Reed and Andrew Bishop will not travel to Spala for the last of two 10-day training camps with the rest of Gatland's training squad.
Gareth Delve, who has just returned from playing for the Melbourne Rebels, former Lion Dwayne Peel and crocked duo Richard Hibbard and Jason Tovey also stay at home.
But Gatland, a Lions coach in 2009, has reassured the players that they are still in contention for the World Cup warm-up matches and the showpiece tournament itself.
"The door is still very much open for those players who are not making this trip," said Gatland.
"This camp will have a strong focus on rugby whilst retaining the skills and conditioning elements of our first visit to Spala.
"It will give us further opportunity to see more of some players in particular in and intense rugby environment, with the likes of Ryan Bevington, Lloyd Burns, Justin Tipuric and Scott Williams all gaining an extra opportunity to show us their worth.
"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.
"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.
Powell, a tourist in 2009, has not featured for Wales since starting in the 19-26 defeat to England in last season's RBS 6 Nations opener.
Thomas played in all but one of Wales' 2011 Championship, while Rees sat out the tournament due to suspension.
Meanwhile two-tour Lion Gethin Jenkins, who missed the entire Championship after foot surgery is back to full fitness.
The Cardiff Blues prop is relishing the opportunity to make up for lost time in Wales' three World Cup warm-up matches against England (twice) and Argentina in August.
"It's important for our final preparation, to get a place in that squad and to get a place on the plane, to do well in Poland," said Jenkins.
"We want to be at our fittest and most powerful, but we also know that unless we can play rugby as well it won't matter a jot.
"The camp has been tough mentally and physically, but the next challenge will then be to perform on the pitch. We will be strong and fit, but we need to convert that into performing in games.
"There is only so much you can learn about a rugby player from a fitness test and these games against England and Argentina will be the opportunity for the coaches to assess the players in a match.
"Those Tests will be the acid tests. Hopefully, if I get my chance, it will go well and I can go to the World Cup, but I'm not taking anything for granted."
Gatland will announce a final 30-man squad on August 22, ahead of their departure for September's tournament on August 31.