Jeff Kenna's driven ball inside took a deflection off a Barnsley defender to sit up conveniently and that was all the invitation Gallacher needed, rifling it past David Watson to inflict a second late defeat in four days on the Yorkshire side.
"The luck isn't going with us, but it will even itself out," insisted their manager, Danny Wilson, afterwards. "There are still seven games to go and we didn't look like a team that's relegated."
It was not all good news for Blackburn, by any means. Apart from not playing particularly well for most of the match, they also lost their England goalkeeper, Tim Flowers, when he landed heavily on his shoulder after punching the ball away.
It was the same shoulder that has troubled him for much of the season and Flowers will have a scan today to assess the damage. "It doesn't look very good," said his manager, Roy Hodgson. "If it needs surgery now he will have to have it."
The change around in goal was emphasised when the first task of his replacement, Alan Fettis, was to pick the ball out his net. It was doubtful whether Flowers would have done any better, however. Georgi Hristov's shot after he had run on to Nicky Eaden's excellent through ball was so crisp and decisive - and in such stark contrast to his sloppy finishing before that.
As Wilson pointed out, Hristov had a splendid opportunity even before Blackburn took their early lead, losing control with the goal at his mercy.
So soon after the tribulations of their defeat by Liverpool on Saturday, Barnsley had cause within minutes to regret not getting away to a flying start.
Martin Dahlin, in the starting line up ahead of Gallacher after the Scot had only recently resumed training after his groin injury, caught their defence flat-footed with a quick angled ball to Chris Sutton. Nobody followed him as he ran for the return and he was able to poke it past Dave Watson and into the net.
Barnsley were generally the better side after that, regrouping impressively after the trauma of their sendings off and assorted pitch invasions at Oakwell at the weekend. Had Hristov's finishing been better, they would have been back on terms long before their breakthrough in the 67th minute.
"He is always capable of scoring goals," said Wilson of his wayward Macedonian. "His ratio isn't what we want from him, but you've got to remember that he's only 21, in a strange country and still adjusting.
"I'm going to be very, very patient with him, because he can be a big asset to Barnsley for years to come."
Unfortunately, his goal also had a reviving effect on Blackburn, especially in conjunction with Gallacher's winning return to action.
Blackburn Rovers (4-4-2): Flowers (Fettis, 64); Kenna, Hendry, Henchoz, Wilcox; Ripley (Duff, 62), Sherwood, McKinlay, Flitcroft; Sutton, Dahlin (Gallacher, 72). Substitutes not used: Davidson, Broomes.
Barnsley (3-5-2): Watson; Morgan, Moses, Jones (Liddell, 89), Eaden; Bosancic, Redfearn, Bullock (Marcelle, 67; Sheridan, 83), Barnard; Hristov, Ward. Substitutes not used: Appleby, Leese (gk)
Referee: M Reed (Birmingham).Reuse content