He notably played in the Premier League for Queens Park Rangers over several seasons, and would have two spells with the Loftus Road club.

Gallen spent the summer of 2001 training with QPR, now in Division Two, with a view to joining on a free transfer, but a compromise on wages could not be reached.

On 11 January 2007 Gallen linked up with his former QPR manager Ian Holloway, joining Plymouth Argyle on loan until the end of 2006–07 season.

On 14 November 2008, Gallen joined League Two side Luton Town on an initial one-month loan,[9] which was later extended by a further month.

At the beginning of January, Gallen had his contract at MK Dons terminated by mutual consent,[10] and he subsequently signed for Luton as a free agent.

[11] Now in non-league football, Gallen found himself utilised back in his familiar position as a striker, scoring eighteen goals in the 2009–10 season, including his first ever hat-trick on 5 April 2010 in a 6–0 win over Grays Athletic.

After a short spell with Leverstock Green in 2011–12, Gallen signed for Aylesbury United of the Molten Spartan South Midlands League Premier Division at the start of the 2012–13 season.

[18] Gallen has appeared on The Magic Sponge podcast by Dave which is hosted by comedians Rob Beckett, Ian Smith and ex-footballer Jimmy Bullard.