Gündoğan is no longer young, his contract only has 1 season left and if Man City does not sell him in the transfer market this season then Gündoğan will leave for free at the end of the season.
Before you see that Pep Guardiola doesn't want to sell Gündoğan, it means Pep Guardiola still needs Gündoğan at Manchester City, and if Gündoğan's contract with Manchester City comes to an end, Manchester City can possibly extend his contract, which will allow Gündoğan to remain at Manchester City, so there's no need for you to get worried about Gündoğan leaving Manchester City for free when his contract with Manchester City comes to an end.