Second half strikes from Carlton Morris and Alex Mowatt consigned Rovers to a third successive defeat as Barnsley ran out narrow winners at Oakwell.
Adam Armstrong did pull a goal back late on, but it was a case of too little, too late for the visitors.
After an even first half, Morris emerged off the bench 10 minutes after the interval and handed the hosts the advantage with a precise strike less than 20 minutes later.
A fine run and finish from Mowatt then made it 2-0 with just a minute of the game remaining before Armstrong's 20th goal of the season ultimately counted for nothing.
The boss made three changes to the side in Yorkshire with Taylor Harwood-Bellis and Elliott Bennett making their first starts of the season.
Elsewhere, Stewart Downing came into midfield in place of Jacob Davenport, whilst Ryan Nyambe and ex-Barnsley man Joe Rothwell were the other pair to drop out.
A smart save from Thomas Kaminski saw the Belgian beat away Conor Chaplin's snapshot inside the opening 10 minutes at a drizzly Oakwell.
At the other end, the returning Bennett played a superb ball over the top towards Gallagher, but the ball didn't bounce kindly for the frontman.
The hosts went close to breaking the deadlock at the midway point of the first half, but his volley drifted just wide of Kaminski's left-hand post.
Moments later and Victor Adeboyejo, one of three changes made by Valerien Ismael, forced Kaminski to tip over from a narrow angle after a quick free kick was played by home skipper Mowatt.
The signs continued to be encouraging as half-time approached for Rovers, with Bradley Dack heading Barry Douglas' cross just over Bradley Collins' crossbar.
With nothing to split the two outfits, the teams went into the break as they started the game, before Tykes chief Ismael made a half-time change by replacing Callum Styles with Jordan Williams.
He made a triple change soon after and one of those alterations almost made an immediate impact; Morris seeing his strike brilliantly blocked by Darragh Lenihan.
Seconds later and Mowatt drove just wide of Kaminski's goal as the opener continued to evade both teams.
However, with 17 minutes remaining, the Tykes took the lead through that man Morris.
The former Norwich City forward had made an impact after entering the action and scored for the second successive game when he curled home via the base of Kaminski's left-hand post.
That resulted in three alterations from Mowbray - Harvey Elliott, Tyrhys Dolan and Corry Evans all coming on in a bid to turn the game around.
But it was Barnsley who went closest to adding to their lead as Morris forced Kaminski into action again with 10 minutes remaining in the contest.
The game was sealed with only a minute remaining as Mowatt raced into a gap in defence, rounded Kaminski and slotted home to wrap up the points.
There was to be one last moment in the game as Armstrong flicked over Collins and in two minutes into added time, but that was to only be a late consolation as Rovers suffered a third successive defeat.
Barnsley: Collins, Styles (Williams, 46), Andersen, Brittain, Woodrow (Dike, 55), Chaplin (Frieser, 55), Sibbick, Palmer (Kane, 64), Mowatt (c), Adeboyejo (Morris, 55), Helik.
Unused substitutes: Walton, Odour, Sollbauer, Moon.
Goals: Morris, 73, Mowatt, 89
Booked: Andersen, 52.
Rovers: Kaminski, Bennett (Dolan, 75), Harwood-Bellis, Lenihan (c), Branthwaite, Douglas, Travis (Rothwell, 82), Downing (Evans, 75), Dack (Elliott, 75), Gallagher (Brereton, 86), Armstrong.
Unused substitutes: Pears, Bell, Davenport, Buckley.
Goal: Armstrong, 90.
Booked: Harwood-Bellis, 82
Referee: Matthew Donohue