Gallen still showed why he is one of the best forwards in the Championship with two excellent goals and is forging an impressive partnership with Paul Furlong who also got his name on the scoresheet.
Manager Ian Holloway stated before the game a victory would keep alive their hopes of finishing in the top six and, on the evidence of this performance, they have every chance.
QPR dominated the opening exchanges and could have opened the scoring after just six minutes.
Gallen broke away down the right before crossing for strike partner Furlong who fired a volley just over the crossbar.
Furlong was involved in the heart of the action once again just two minutes later when Martin Rowlands' cross picked out the experienced forward inside the box, but his close-range header was well-saved by Hornets keeper Richard Lee.
Watford forced their way back into the game and came close to opening the scoring in the 17th minute.
Johnnie Jackson, on-loan from Tottenham, fired the ball across goal and defender Marcus Bignot just managed to clear the ball over his own crossbar.
But it was the home side that had a golden opportunity to get themselves in front on 20 minutes after Gallen's shot was handled on the line by Brynjar Gunnarsson.
Referee Graham Poll had no hesitation in brandishing the red card and awarding a penalty, but Gallen saw his spot-kick superbly saved by Lee. The rebound fell to Gallen whose header came back off the crossbar and when the ball bounced back down, Rowlands could only direct his header over the bar.
But QPR finally got their just rewards when Furlong broke the deadlock on 34 minutes. Lee Cook's cross eventually fell to the former Chelsea forward who made no mistake this time striking the ball into the corner of the net.
And the home side made it two on the stroke of half-time when Gareth Ainsworth caused all kinds of problems for Jermaine Darlington down the right.
He pulled the ball back for Gallen who side-footed the ball past Lee into the back of the net.
And Gallen added his second goal of the game and QPR's third on 57 minutes. Cook was instrumental once again in setting up the goal when his cross from the left picked out the experienced forward who headed the ball into the corner of the net.
Watford pulled a consolation goal back on 79 minutes when substitute Neal Ardley planted a clever lob over Simon Royce, but it came too late to stage any kind of comeback and they will be anxiously looking over their shoulders at the bottom of the table.