The 26-year-old made 192 appearances for QPR during an eight-year spell at the club before joining Huddersfield Town.
It is understood that Rangers boss Ian Holloway tried to sign Gallen during the summer when his contract with The Terriers was allowed to expire.
"Kevin's a quality player and I'm getting him on a free transfer," he exclaimed.
The forward's departure has prompted speculation that Parkin already has a replacement in mind and, in spite of rumours linking him with a move for Mansfield's Chris Greenacre, a raid on his former club Rochdale would look a more likely option with both Clive Platt and Paul Connor currently in excellent form for the Spotland outfit
