Forwards Jonathan Thomas, Andy Powell, Scott Andrews and Ken Owens, who did not take part in Wales' second Poland training camp this month, have returned to the fray.
Scarlets back-row forward Rob McCusker misses out through back problems, while the other absentees from Gatland's preliminary party announced in early June include scrum-half Richie Rees, fly-half Jason Tovey, centre Andrew Bishop and hooker Richard Hibbard, who is recovering from injury.
Gatland, though, has not ruled out selecting any player for further warm-up fixtures against England in Cardiff on August 13, and at home to Argentina seven days later.
He plans to announce his final 30-man squad on the tournament cut-off date of August 22.
Gatland said: "The team to face England at Twickenham will be selected from this training squad.
"We have established a nucleus of players who will form our World Cup squad based on their performances in training over recent weeks, but it is now time to translate those efforts on to the field.
"The players have worked hard, but places in that final World Cup squad are still very much up for grabs and a lot will depend on what we see over the course of these next three games.
"We have not shied away from facing top-quality opposition in preparation for this World Cup, and it doesn't come much tougher than facing the 2003 World Cup winners and 2007 finalists on their home patch.
"The warm-up fixtures will be tough, but we are in a really tough group at the World Cup, and replicating those conditions can, ultimately, only be of benefit to us."
Wales kick off their World Cup campaign against holders South Africa in Wellington, New Zealand on September 11.
Wales squad: Forwards - S Andrews (Cardiff Blues), G Jenkins (Cardiff Blues), R Bevington (Ospreys), A Jones (Ospreys), P James (Ospreys), C Mitchell (Exeter), M Rees (Scarlets), H Bennett (Ospreys), L Burns (Newport Gwent Dragons), K Owens (Scarlets), B Davies (Cardiff Blues), L Charteris (Newport Gwent Dragons), A-W Jones (Ospreys), R Jones (Ospreys), D Lydiate (Newport Gwent Dragons), S Warburton (Cardiff Blues), M Williams (Cardiff Blues), J Thomas (Ospreys), J Tipuric (Ospreys), J Turnbull (Scarlets), T Faletau (Newport Gwent Dragons), G Delve (Melbourne Rebels), A Powell (Sale Sharks).
Backs - L Byrne (Clermont Auvergne), L Halfpenny (Cardiff Blues), M Stoddart (Scarlets), G North (Scarlets), S Williams (Ospreys), A Brew (Newport Gwent Dragons), J Roberts (Cardiff Blues), J Hook (Perpignan), J Davies (Scarlets), S Willams (Scarlets), G Henson (unattached), S Jones (Scarlets), R Priestland (Scarlets), M Phillips (Bayonne), L Williams (Cardiff Blues), T Knoyle (Scarlets).
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