Former Manchester City captain Gundogan told CBS: "I am disappointed (with the result). Very disappointed actually as we were in such a good position.
"Not just after the first leg, but after the first goal we scored. Everything was in our hands and we just gave it away in the most simple of manners."
He added: "We just gave it to Paris Saint-Germain and that's the most disappointing part."
Gundogan then continued: "If he (Araujo) fouled him (Bradley Barcola), I guess it's a red card. I haven't seen the replay, I don't know. You guys probably know better than me.
"It's difficult to say. You know, in these kind of crucial moments you need to be sure to get the ball.
"If you don't (whether you're going to get the ball), and again I don't know if he touched the ball or not, you need to stay away.
"I would prefer to concede a goal there or give the striker 1v1. Although, the ball was played quite far ahead so I don't even know if he would have reached the ball.
"But, maybe let our goalkeeper save us or even concede a goal because to go a player down so early on just kills your game."
Asked about Vitinha's goal, he added: "Again, it's another mistake. We have to step out there. We can't let him shoot because he was so free.
"It wasn't anything we haven't trained before. The closest player to the ball has to step out. We were three with three and Vitinha was the fourth man. Someone has to step out.
"No one did or they did it too late. It was a good shot and they got their second goal. But, it wasn't necessary at all. It's too late right now."