City fell behind to Raffael’s drilled strike midway through the first half but De Bruyne’s cross just before half-time was turned home by David Silva to ensure City will finish second in Group C.
Despite controlling the second period, City rarely threatened to take the lead and both Gladbach captain Lars Stindl and Man City midfielder Fernandinho saw red after picking up two bookings.
However, De Bruyne, City’s man of the match, insists the only aim of the Blues was to reach the first knockout round.
He said: “It was always about the result. After we got the red card it killed the game but obviously the result was the most important.
“It wasn’t nice to see at the end of the match but now we’re through. They were very compact. Even before they scored I think they just wanted to defend.
“It was a good goal after a tough first half but then the second half was just about controlling.”
Ilkay Gundogan insisted that City’s reaction to falling behind to the German club shows their character and everything they are learning under manager Pep Guardiola.
He said: “This is the football we want to play. We always want to react when we recognise that things aren’t going well. We tried to change and the manager helped us every time which was necessary so that we could control the game.
“It was tough. We knew that Borussia are a strong team and we definitely saw that in the first half. We controlled the second half, especially after Fernandinho’s red card when we just held the ball. It was necessary to do that.”
City’s final match in Group C of the Champions League comes against Celtic on Tuesday 6 December.