Gallagher joins the Jets from Vietnamese Club Hanoi T&T, where he made 11 appearances during the 2014 V-League season, scoring two goals. The 23-year-old graduated from the youth team of Sydney FC before having stints at Hyundai A-League Clubs Central Coast Mariners and Melbourne Victory. Gallagher also represented Australia at U-20 level on 18 occasions, playing alongside current Jets teammates Mark Birighitti and Ben Kantarovski at the FIFA U-20 World Cup in 2011.
