Initially a half forward, Gallagher hailed from Rosebud. His 34 goals in 1957 was enough to top the goal-kicking at Essendon, helped by a haul of 12 goals and four behinds from full-forward in their round eight win over Geelong at Windy Hill. In doing so he joined John Coleman and Ted Freyer as the only players to have kicked 12 goals or more in a game for Essendon. He also booted three majors in the 1957 VFL Grand Final where he was his club's only multiple goal-kicker in a 61-point loss to Melbourne. With his place in the side no longer assured in 1958, Gallagher opted to join New South Wales club Turvey Park as captain-coach.