The Crystal Palace midfielder, who is on loan from Chelsea, has starred under Patrick Vieira this season.
The 21-year-old has four Premier League goals and two assists in 10 games this season and has thrived in the Eagles side.
Gallagher has been elevated to Gareth Southgate's squad from the Under-21s with Jordan Henderson, Jack Grealish, Raheem Sterling, Mason Mount and Luke Shaw all not making the trip to play San Marino.
Henderson and Grealish have returned to their respective clubs for further assessment on injuries.
Sterling misses out due to a personal matter while Mount (dental surgery) and Shaw (concussion) will now not link up with the squad.
Gallagher featured for the Under-21s on Thursday, but now is in contention to start in the World Cup qualifier at San Marino on Monday.
Former England striker Dean Ashton believes the midfielder was deserving of the call from Southgate.
"He absolutely deserves his call up. I'd expect him to start. I'd expect him to come over and start the game, I'd be surprised if not," Ashton told talkSPORT.
"It's such a great opportunity for Gareth Southgate to give these players the opportunity to play with the freedom against a side they are going to beat.
"Let's be honest, San Marino's record is so abysmal that every player should go out with no pressure.
"What a great opportunity to experience a celebration of an evening in what should be getting to a World Cup.
"I'm old enough to remember them not getting to a World Cup."