Lions captain Danny Shittu headed the only goal of the sixth-round replay against Blackburn at Ewood Park to secure a Wembley encounter against Wigan.
Shittu marshalled a Millwall defence that shut out Rovers for the second time in four days and Gallen believes such solidity can act as a foundation for moving more than five points away from the relegation zone.
"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."
Shittu, who added to a fourth-round goal against Aston Villa, claimed man-of-the-match honours as he did on Sunday.
Gallen was full of praise for his endeavours, but revealed fitness concerns over the 32-year-old powerhouse prevent him from being used as a regular set-piece battering ram.
"He's been exceptional this year," Gallen added. "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."
A sparse crowd of 8,635 - the lowest FA Cup quarter-final attendance in recent memory and 100 fewer than the number at Chesterfield's last-eight clash against Wrexham in 1997 - could not inspire a Blackburn fightback.
Leon Best's 63rd-minute attempt, brilliantly cleared off the line by Shane Lowry, marked a rare moment of inspiration and Rovers manager Michael Appleton conceded morale was through the floor at full-time.
"You can probably imagine the atmosphere in the dressing room after the game, he said. "It's the type of situation that, as a manager, you don't forget.
"To loose how we did lose, to a set-play, probably makes it even harder to take.
"We huffed and puffed, the lads gave us absolutely everything to be fair.
"I had to make a lot of changes again due to injuries, which was hard, but I can't ask any more of them.
"From an effort point of view, there's nothing more that any of the players could have given me.
"From a quality point of view, yes we could have been better."
Blackburn's form sheet since their fifth-round upset of Arsenal reads played seven, lost five, drawn two and they could head into Sunday's east Lancashire derby against Burnley three points above the drop zone.
A second consecutive relegation for the 1995 Premier League champions is an unthinkable prospect and Appleton believes a huge change in attitude is required among his squad.
"We've got 10 massive games now," he added. "There's an opportunity to change the whole mentality of the football club.
"For probably two or three seasons now it's been acceptable to lose and it's not too much of a bad thing, and that's the hardest thing for me to take - for some of the players it doesn't hurt as much as it should do.
"I'm not saying that they don't hurt, but it doesn't hurt as much as it should do."