City left it late as striker Sergio Aguero scored a 90th minute penalty to give the English side their first Champions League win of the season.
Hart played a crucial role to help City bounce back from their defeat by Juventus last month, saving a 20th minute penalty from Raffael and producing several brilliant stops to frustrate Gladbach in the Group D contest.
"We didn't play well... We're not really happy with the performance but winning puts us in a good position," Hart told the club website (www.mcfc.co.uk).
"We were really disjointed, like in the past couple of Premier League games, we had a good half and a bad half.
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"I think we deserved to win. We profited when they were a little bit tired at the end. They made a few mistakes and we kept our quality," the 28-year-old added.
Hart also praised Aguero, who was the top goal-scorer in the Premier League last season, saying that the Argentine can score almost at will.
"We left it late but Kun's a cool guy and no matter whether he's in form or not, he will score," the England international said.
Manager Manuel Pellegrini also heaped praise on Aguero.
"It's not easy to score a penalty in the last minute, we know how important Sergio is for our team," said the Chilean, who was also pleased with his team's reaction after being a goal down.
"They played better than us, especially defensively because we didn't defend well, we had a strong reaction for the last 30 minutes and finally we won the game. I must say Borussia played very, very well," the former Real Madrid manager said.
(Reporting by Shravanth Vijayakumar in Bengaluru; editing by Amlan Chakraborty)
