The 21-year-old midfielder, who is on loan at Selhurst Park from Chelsea, was rewarded by England manager Gareth Southgate after scoring four goals and adding two assists for Palace this season.
Gallagher’s call comes as Southgate faces several absences ahead of Monday’s World Cup qualifier in San Marino, a game England only need a point to secure their place at next year’s tournament.
Jordan Henderson and Jack Grealish have returned to Liverpool and Manchester City respectively for further injury assessment, while Raheem Sterling will miss the game for personal reasons.
Mason Mount and Luke Shaw, who missed Friday’s 5-0 win over Albania, will no longer join the squad. The Chelsea Mount midfielder continues to suffer from the after-effects of dental surgery, while Shaw recovers from a concussion.
Gallagher, who has represented England at all levels from the Under-17s, took part in Thursday’s European Under-21 Championships qualifying victory over the Czech Republic.
England U21 boss Lee Carsley called Leeds forward Sam Greenwood to his squad ahead of their game against Georgia on Tuesday.
This is late breaking news which is being updated and more details will be released shortly. Please refresh this page for the latest updates.
Sky Sports brings you live updates as they happen. Get the latest sports news, analysis, exclusive interviews, reruns and highlights.
Sky Sports is your trusted source for sports news headlines and live updates. Watch live coverage of your favorite sports: Football, F1, Boxing, Cricket, Golf, Tennis, Rugby League, Rugby Union, NFL, Darts, Netball and get the latest news on transfers, results, scores and more .
Visit skysports.com or the Sky Sports app for all the sports news headlines. You can receive push notifications from the Sky Sports app for the latest news from your favorite sports and you can also follow @SkySportsNews on Twitter to get the latest updates.