Gallagher made his senior competitive debut for Saracens in the Anglo-Welsh Cup against Welsh side Ospreys in November 2014.

[2][3] He was voted the Saracens Academy Player of the Year following strong performances for Saracens Storm during the 2015–16 Premiership Rugby A-League, as well as helping loan club Old Albanian secure promotion to National League 1 after winning the 2015–16 National League 2 South play-offs.

[1] He joined Bedford Blues as a dual-registered player ahead of the 2016–17 RFU Championship season, though he was soon called into action for Saracens.

[7] His last appearance for Bath saw him start in the 2024 Premiership final which they lost against Northampton Saints to finish runners up.

[8] Gallagher signed with Italian United Rugby Championship club Benetton a three-year contract from the 2024–25 season.