Millwall captain Danny Shittu headed the only goal of the sixth-round replay and the Lions then denied Blackburn a goal for the second time in four days to set up a Wembley clash with Wigan.
Gallen heaped praise on his side's back-line and challenged them to take this form into the Championship as they attempt to halt their slide down the table.
"We're delighted to win, I know it means we get through to the semi-final but we won 1-0 and we haven't been winning many games," he said.
"We didn't concede, which is a good start because our league form is not very good.
"The back four, with Jack Smith (in front of them) and (goalkeeper) David Forde - if you look at those six, I thought that in these two games they were pretty outstanding.
"One of the biggest things with getting through to Wembley is the motivation for the players who want to be in that team - they're going to have to perform in the league games.
"Our league form is pretty alarming. We're sliding down the table fast and everyone below us keeps winning.
"On 44 points we need at least three more wins, maybe four more wins to secure our status in the Championship which is always the big aim at Millwall."
Gallen believes Shittu's goal was a fitting reward for his impressive displays this season.
"He's been exceptional this year," Gallen said. "Sometimes we're a bit worried about his fitness and worried about his body a little bit.
"Getting him up for corners - we haven't been getting him up all the time, but once you do get him up then obviously he's a threat because that's his second goal from a corner in this competition."