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."