Dutchman Robben's rocket-like volley from the edge of the box on 74 minutes brought the score on the night back to 3-2 and 4-4 on aggregate which was enough to see Bayern qualify on the away goals rule.
NEWS:Robben stunner sends Bayern through
VIDEO:Robben's recent wonder-goals
"The goal from Robben was unbelievable," said van Gaal.
"The first half wasn't very good and we had to improve. It's incredible what we did in the second half - we played fantastically.
"It's not always easy to play against 10 men. We played like a great team."
United raced into a 3-0 lead and a 4-2 lead overall before Bayern replied with a goal before half time from Ivica Olic and then Robben's clincher.
Robben told German TV: "When we were behind 3-0 I thought it was going to be a very difficult evening. But 3-0 was the same as 2-0. We had to score a goal.
"The goal from Olic gave us faith that we could still advance. We have to be honest. They were the better team at the start, with the atmosphere.
"I saw that Franck (Ribery) saw me at the corner. I knew the ball would come across to me. I tried to stay calm. You can use force or technique. I tried to use technique. It worked out."
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