The Lions moved up to seventh, two points behind sixth-placed Nottingham Forest, courtesy of goals from young striker John Marquis.
"We came up from League One last season via Wembley," said Gallen.
"No-one really expected us to be challenging again this season.
"We would have accepted, I suppose, consolidation this time, but we always felt there was a chance we might be able to challenge for the play-offs again.
"That is what has happened. There is no pressure on us. We have just got to go into the final two games of the season trying to win them and see where it takes us."
Source: PA
Source: PA