The Germany international has admitted that he still finds it difficult to come to terms with the fact that he missed Brazil 2014 because of a back problem
The Germany international aggravated a back problem in a friendly against Paraguay last August and suffered a succession of setbacks during his rehabilitation, ultimately ruling him out of his country's triumphant Brazil 2014 campaign.
"I would have been there and now I would have been world champion," Gundogan told Welt am Sonntag.
"I missed this great tournament and that's not a nice feeling. I've lost a whole year."
Gundogan added, though, that while he was enforced to endure numerous dark days during his recovery, he is now feeling like his old self again as he nears his return to full fitness.
"I just didn't want to go out of the house," the 23-year-old said of his spell on the sidelines.
"My thoughts revolved around when I would be able to be fit again. I couldn't enjoy anything else in life.
"But I've not been as happy as I am now in a long time. It's fantastic to be able to train with the team again, even if I'm only restricted to running exercises."