[9][10] Gallacher made his first team debut on 10 August 2014, coming on as a 67th-minute substitute at Ibrox Stadium in a league match against Rangers, replacing the injured Neil Alexander[11] in a 2–1 win.

[13] On 14 January 2016, Gallacher signed a contract with Alloa until the end of the season, after leaving Hearts by mutual consent.

[14] Gallagher signed a one-year deal with St Mirren in June 2016,[15] beginning the season as the Saints first-choice goalkeeper.

After Jack Ross his former Alloa manager was appointed manager of st mirren in October 2016, Gallacher was surprisingly dropped after man of the match performances in previous two games in favour of Jamie Langfield, however, he returned to the starting line-up in mid-November, playing in St Mirren's 2–1 victory over Ayr United in the Scottish Challenge Cup.

He regained his position after the long absence and kept numerous clean sheets until the season was cut short due to COVID-19.