Also included is Day's new teammate at the Rec, Rhys Priestland - one of three fly-halves alongside Dan Biggar and Bristol youngster Matthew Morgan.
Northampton Saints winger George North, who won his 50th Welsh cap on Saturday, takes his place in the squad alongside Harlequins-bound centre - and fellow British & Irish Lion - Jamie Roberts.
Exeter Chiefs prop Tomas Francis makes the cut after impressing Gatland during the Welsh training camps and links up with former Bath man Paul James in the front row.
There is, however, no place for Gloucester Rugby back rower Ross Moriarty, who made his Test debut this month.
Wales kick off their World Cup campaign against Uruguay in Cardiff on September 20 but things will soon get tougher, with Fiji, England and Australia also in Pool A.
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