In their ninth match in four weeks, the last thing they needed was an erratic performance from their goalkeeper, Neil Sullivan, but that is exactly what they got, starting with the goal he presented to Gallacher after only six minutes.
Collecting a routine back header from Alan Kimble, Sullivan somehow let the ball squirm from his grasp and Gallacher needed no further invitation.
Sullivan also had a helping hand in Gallacher's second, although this time it was harder to lay all the blame at his door. A fine diagonal ball from Graeme Le Saux found Blackburn's new Danish striker, Per Pedersen, bearing down on goal and Sullivan did well to parry his shot. Unfortunately, he parried it straight into the path of Gallacher.
Rovers were carving a lethargic Wimbledon apart at will. But there was a flicker of hope for them in a sequence of events that could have turned the match. Pedersen failed to score with a simple header from Tim Sherwood's cross and, within a minute, Colin Hendry's miskick at the other end allowed Marcus Gayle to centre for Efan Ekoku to score. It should have been 3-0 but suddenly it was 2-1.
However, any prospect of Wimbledon escaping with anything from this game disappeared 11 minutes into the second half. Gallacher scooped an instinctive pass out to the flank for Pedersen and then sneaked into the penalty area for the return. When it came, he volleyed sweetly past Sullivan.
Blackburn could have won even more comfortably with some steadier finishing from Gallacher's team-mates and if the game had not become so staccato in the second half. When one of the referee's assistants had to replace his flag five minutes from time it was presumably because it was worn out.