Eligible for both Scotland and the Republic of Ireland, Gallagher instead opted to represent England at youth level, doing so at Under-17s, Under-18s, Under-19s and Under-20s.
He was part of England’s triumphant Under-17s World Cup campaign alongside fellow recent signing Marc Guéhi, featuring in the final victory over Spain as the Young Lions were crowned champions.
In 2019 he received his first call-up to the Under-21s side, making his debut against Slovenia in Maribor – he has made nine appearances for the team since.
After signing a contract at Chelsea in 2018, Gallagher was an unused substitute for the 2019 Europa League final in Baku, where he received a winners’ medal after Chelsea’s win over Arsenal. It capped a fine year for the youngster, having won the club’s Academy Player of the Season award.
A loan move to Charlton Athletic allowed Gallagher to make his professional debut, featuring 26 times across a successful half-season spell. After three goals in his first six games for the Addicks, he was named the Football League’s Young Player of the Month on his arrival.
After being recalled from the Valley, Gallagher made the move to south Wales to join Swansea City for the remainder of the season. Alongside Guéhi he formed part of an exciting young side, registering seven assists as the Swans reached the play-offs.