The young Scotsman moves from Blackburn Rovers to Plymouth after signing emergency loan papers.
Gallagher said: "I've come here to play first-team football and help the club move forward.
"I met the manager and everything he said made sense and I had no hesitation in agreeing to sign."
Gallagher enjoyed his first Pilgrims run-out in the 0 - 0 draw today, adding: "I enjoyed the game. You can't beat the feeling of getting back playing again. I had a chance right at the end of the first half that I should have done better with but a point away from home is a good result - especially against Burnley, as they are a good team.
"I'm going to enjoy coming to Plymouth and - hopefully - getting a good run in the team."