The 25-year-old moved to the Etihad Stadium from the Bundesliga giants in the summer transfer window for a reported £20m fee.
The Germany international, who signed a four-year deal with City, is determined to prove his ability away from Dortmund, but would consider moving back to the Westfalenstadion at some point.
"If it was the case in the next one or two years, I would probably not be happy about it," Gundogan told Bild. "Because it would probably mean that I have failed or not developed myself.
"If I'm 29 or 30, I do not know if [Dortmund would] still want me. Basically, I would not exclude the possibility, but now I have other big targets."
After recovering from a knee injury, Gundogan scored his first Premier League goal in a 4-0 rout over Bournemouth.