The Lions slipped to a 1-0 defeat at Derby, but results elsewhere kept them up.
Huddersfield's equaliser at home to Barnsley and Crystal Palace's late fightback against Peterborough - with Posh ultimately joining Wolves and Bristol City in the relegation zone - ensured Millwall will again be in the Championship next season.
"The plan was to come here and at least get a point; unfortunately we've relied on other teams," Gallen said.
"It's a great achievement to stay in the division - we're one of the smaller clubs in the Championship. We haven't lost a London derby this season and that's something important to our fans."
Kenny Jackett's side made headlines for all the wrong reasons after violent scenes among their fans during their FA Cup semi-final defeat to Wigan.
And Gallen said the negative publicity had an impact on their squad, which he says will need strengthening next season.
"The media fallout from the FA Cup semi-final put us under pressure," he said. "I think it's obvious where we need to improve: we need strengthening in creativity and goal-scoring positions, although I'm aware offensive players cost a lot of money."