The Reds resurgence was stopped in its tracks by a 4-1 win for the Lions in the Championship''s surprise result of the day.
"It''s a massive achievement to come to a place like this, score four goals and beat Forest," said Gallen.
"When you see the team that Forest put on the pitch they are very impressive and then you get more worried at the quality they have to come off the bench."
Home boss Sean O''Driscoll admitted he did not see the result coming but praised the impact of Millwall''s front pairing for changing a game that had been ''50-50'' at the break.
Source: Forest MAD