Plymouth's Akos Buzsaky looked like he would be leading the Pilgrims to their second away win of the season after a super 30-yard strike in the first half, but Gallen stepped forward to kill their hopes.
Rangers dominated the game for long periods, but lacked the killer instinct to kill the game off.
Striker Marc Nygaard missed two headers inside the first ten minutes and winger Lee Cook also had a good opportunity, but his end product left much to be desired.
The visitors sat back and absorbed much of QPR's early pressure and then hit their hosts on the break.
Their tactics paid off and Plymouth took the lead after 39 minutes. Paul Wotton found Buzsaky in midfield and the Hungarian International struck a perfect drive into the corner of the net.
Wotton could have doubled the lead a minute later, but his close-range effort deflected wide.
After half-time Paul Furlong nudged a chance just wide of the post.
On 59 minutes Nygaard was left furious after referee Matt Matthias waved away a penalty claim after the forward appeared to be pulled back by Mathias Doumbe.
Ian Holloway's men were getting frustrated, but eventually they got the breakthrough they were searching for.
Plymouth's Scott Taylor hand balled in his own area and this time the ref had no hesitation in pointing to the spot.
Gallen stepped forward to convert the kick and bring Rangers level with a well-taken sidefoot.
The goal lifted the home side and Furlong came close again soon after.
With ten minutes left substitute Gareth Ainsworth hit a half-volley which flew just over the bar and in the final minute Marc Bircham's long-ranger was easily dealt with.
Plymouth almost snatched an injury time winner, but Bjarni Gudjonsson's late strike was saved by Simon Royce.