Both sides had nothing to play for and it showed in the topsy-turvy nature of the game.
But John Swift's 15th-minute free-kick gave the shellshocked Royals hope.
That seemed to be extinguished early in the second half when Joe Rothwell's deflected strike restored the two-goal cushion, but Sam Baldock was introduced off the bench and in the space of five second-half minutes, had headed home his sixth of the season to give Reading hope, and set up Yakou Meite to notch an equaliser – his 17th goal this term.
Gallagher's late header then proved decisive as Blackburn exceeded last season's points total.
Hayden Carter made his first senior appearance for Rovers, meaning Tony Mowbray's starting XI contained five academy graduates.
Blackburn flew out of the traps and were ahead within three minutes as poor defending allowed Brereton far too much space and he cut inside to slot home from close range.
They doubled the advantage three minutes later when Rothwell found Armstrong just outside the box and the in-form striker engineered space to rifle home a left-foot effort into the bottom right corner – his 16th goal this season.
Mark Bowen warned his team not to go through the motions, and they responded in the 15th minute through Swift, who thumped a sumptuous 25-yard free-kick up and over the wall, giving Christian Walton no chance.
Tony Mowbray's side started the second half brightly and deservedly scored a third in the 56th minute as Rothwell's deflected shot deceived Cabral and found the bottom corner.
Brereton hammered wide when presented with a chance to make it four, and that profligacy was punished in the 64th minute as substitute Baldock announced his presence with a sublime glancing header at the near post.
Incredibly, the visitors were level four minutes later. Baldock again was key, finding space to chip the perfect ball to the back post and Reading's top scorer Meite buried his header into the far corner.
He even found time to hammer the woodwork before the end as Rovers deservedly took home maximum points.