An enthusiastic Arsenal football supporter, he was introduced to rugby union at his childhood school, St. Joseph's Academy, Blackheath, where he was required by the headmaster to join the team.
[3][4] In 1990, Gallagher was one of the first of a succession of All Blacks to switch codes to rugby league, following his understudy Matthew Ridge, who signed with the NSWRL's Manly-Warringah Sea Eagles only a week before.
He was targeted by opposition players in defence and was badly injured in an alleged spear tackle, playing away at St Helens involving Tea Ropati.
He may also have found the tactical differences between the two codes difficult to adjust to, particularly in his favoured full-back position and the move was unsuccessful.
Perhaps his finest moment for Leeds came in a match against Australia during the Australian teams 1990 Kangaroo tour when he scored the opening try of the game at Headingley.
Gallagher's defense came into question later in that game when giant Kangaroo second rower Paul Sironen scored two tries, both times running over the top of him.