The Germany midfielder's contract is due to expire in June 2017 and Dortmund are widely expected to cash in on the influential midfielder if he does not sign a renewal.
Goal revealed on Wednesday that City have pulled ahead of Barcelona and Juventus in the race to sign the 25-year-old midfielder, although talks have yet to be finalised.
After Gundogan's uncle and representative Ilhan was spotted meeting with City sporting director Txiki Begiristain this week, Tuchel admitted his disappointment at the speculation.
READ MORE | Man City lead Gundogan race
"I do not understand what the fuss is all about," he said, when asked about the meeting ahead of Dortmund's Europa League clash with Tottenham.
"We also start planning for next season nice and early. That's the way things work in football.
"I am not disappointed about what's happened. Am I convinced that he will stay put? I will only be convinced he will stay once he signs a new deal. If not, we will have to deal with it.
"Gundogan knows why he is here and how he can develop with Dortmund, I would be amazed if things would change in that regard."