The former Blackburn Rovers trainee originally signed for North End under Paul Simpson in 2007, but a permanent move failed to materialise and he went on to earn promotion to the Premier League with Stoke City.
The Scot was purchased for £1m by Leicester City and played nearly 150 games for the Foxes, but jumped at the opportunity to return to Deepdale under Simon Grayson - with whom he was in the same squad at Blackburn at the beginning of his career.
Gallagher extended his second loan deal until the end of the 2013/14 season and played a key part for Preston as the Club reached the Play-Offs, scoring an impressive 10 goals in 35 appearances.
In the summer of 2014, Gallagher agreed to re-join North End on a season-long loan for the entire 2014/15 season, a fruitful decision as he ended the campaign as the league’s top assist maker, including creating two of the goals in the Play-Off Final at Wembley.
After a successful return to the Championship with North End, Gallagher picked up the supporters' Player of the Year award at the end of the 2015/16 campaign, as well as collecting the Club's Goal of the Year award for a strike against Charlton Athletic.