Lions skipper Danny Shittu headed the only goal of the quarter-final replay at Ewood Park to secure a Wembley semi-final against Wigan.
Millwall prevailed before 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.
Gallen was surprised by the pitiful number but praised 574 travelling supporters who proved to be a vocal minority.
"They were terrific but that's the one thing you know about Millwall - the atmosphere is always fantastic," Gallen said.
"You get 300, 400 or 500 of our fans in any ground and they just make the atmosphere.
"They're just a phenomenal bunch. At Charlton on Saturday there'll be 3,000 of our fans and 3,000 Millwall fans can cause a lot of noise - among other things."