Gundogan worked under the now-Liverpool boss at Signal Iduna Park between 2011 and 2015.
Writing for The Players' Tribune, Gundogan said: "I once managed to make Jurgen Klopp angry. Like, really angry. It was in my second season at Dortmund. We were trailing in the Bundesliga, but we had a shot at the Champions League.
"An hour before training, just to be sure, I asked the doctor to take a look at my hamstring.
"Jurgen came in and he was not happy. I was trying to find a way out of it, even though I knew I was wrong. Jurgen kept saying that we couldn't take any risks. I kept saying that I could train.
"And then he snapped. You know when he gets these intense eyes and grits his teeth?
"He gave me that look and shouted, 'DO WHATEVER THE F*** YOU WANT TO DO!' Then he slammed the door behind him.
"About half an hour later, I put my boots on and walked on to the pitch. Jurgen came up next to me. I was expecting a lecture, but he put his arm around me.
"He said, 'My friend, do you know why I was so angry? I just care about you. And I don't want you to get injured.' Then he gave me a big hug.
"I was shocked. We had had this fight, and now he was talking to me the way a father might talk to his son.
"That showed me what kind of person he is: very emotional, of course, but also very open and honest.
"Jurgen taught me a lesson that day: Always try to be honest. Both with others and yourself."
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