Seth Johnson's super first-half strike had put United in control, but after Derry's dismissal, Gallen stepped forward to rifle home an 85th minute leveller.
There were few chances before Leeds opened the scoring through Johnson's stunning effort.
On 24 minutes Rangers failed to clear a Gary Kelly corner and Johnson pounced on to a loose ball 25-yards out to send a sweet strike beyond Chris Day and into the far corner.
Rangers were struggling to create an opening and things turned worse for them as top goalscorer Paul Furlong limped off after only 35 minutes.
Jamie Cureton took his place and almost forced a leveller on the stroke of half-time. The forward nodded back across goal after a deep cross from Lee Cook, but former Loftus Road favourite Clarke Carlisle cleared the danger with a goalline header.
After the break, QPR stepped up a gear and Gallen was unlucky not to score as his half-volley fizzed just wide of the post.
Ten minutes later Gallen proved a danger again with a testing cross, but Carlisle took care of the threat once again as he headed clear.
Leeds were then left with ten men on 71 minutes as Derry flicked a hand out at Gallen right in front of the referee, Ray Olivier, and Rangers eventually took advantage of the extra man.
Gallen cut inside and fired a shot at goal which Sullivan parried and the striker followed up quickly to hit an unstoppable drive into the far corner.
Gallen had a shot tipped on to the post in the dying stages and Rangers' Adam Miller was also sent off for a second booking as the game ended level.