The German international has recently returned to training following his lengthy lay-off and has admitted that his mood his lifted in recent weeks.
"I've not been as happy as I am now in a long time," he is quoted as telling Welt am Sonntag by BT Sport. "It's fantastic to be able to train with the team again, even if I'm only restricted to running exercises."
Gundogan also admitted that there were times last season when he felt very low.
"I just didn't want to go out of the house," he said. "My thoughts revolved around when I would be able to be fit again. I couldn't enjoy anything else in life."
Gundogan's injuries meant that he missed Germany's victorious World Cup campaign in Brazil this summer.