Gallagher has had a fine debut season playing first-team football and been one of the Championship's rising young stars.
Charlton fans were gutted when Chelsea recalled him in January to send him to Swansea.
“I had a great time at Charlton. It was a homely club and I made a lot of great friends there. Hopefully they can stay up, they had a lot of injuries and problems with the ownership, so had a lot to contend with.
“But things have been good at Swansea — and all the focus is now on winning the remaining games to hopefully push this club back into the Premier League."