Saints striker Gallagher was handed another start for Rovers, as they followed up their 4-2 home victory over Rotherham with another three points, which saw them climb out of the relegation zone.
Gallagher, who has already netted three times for his loan club, turned in another lively display up front, and was unfortunate not to be awarded a penalty early on, when he was brought down in the area by Derby keeper Scott Carson.
Gallagher left the action after 66 minutes, when he was substituted with the score 0-0.
Shortly afterwards, Rovers fell behind when Matej Vydra scored, but they quickly turned the game on its head, with Marvin Emnes equalising immediately before Danny Graham netted three minutes later.
Blackburn are back in action tomorrow night, as they host Sheffield Wednesday at Ewood Park.
Elsewhere, defender Jason McCarthy helped Walsall extend their unbeaten streak to three matches.
McCarthy played the full match as the Saddlers earned a 1-1 draw at Peterborough, with Simeon Jackson’s early striker being cancelled out by Marcus Maddison.
Walsall, who are 18th in the third tier, are in action tomorrow night, as they host leaders Scunthorpe.
In League 2, midfielder Dom Gape played the full 90 minutes, but couldn’t prevent Wycombe from losing 1-0 at Carlisle, with Shaun Miller’s second half goal separating the sides.
Wycombe, who are 21st in the table, host Crewe tomorrow night.
Out in Spain, goalkeeper Paulo Gazzaniga was an unused substitute, as Rayo Vallecano drew 1-1 away to Reus Deportiu, with the result leaving them 14th in the Segunda Division.