Gundogan is very grateful to Pep for giving him the opportunity to play in City. He has won most of the teophies with City and he believes he deserves more. He does not see Galatasaray as a club matches his status and that will make him still stay in City. However, I doubt that Barcelona will take him back when his contract expires with City.
I think Guardiola would have gladly taken Gundogan back at City because at that time their midfield was weakened, Rodri was injured and De Bruyne was only starting to recover after his injury. So Guardiola was in a difficult situation and Gundogan arrived just in time. I do not think he will return to Barcelona because Flick is building a young team and Gundogan is unlikely to fit into his plans. I believe he would not mind staying at City for a while longer if he could but Guardiola will be rebuilding the team anyway.
Off course that is what every coach on their right sense will do because there is no other choice than to do that at that point in time. Gundogan is a nice player even before he went to Barcelona but you are right, Barcelona looks like a young boys team so it is possible that Gundogan may not be a good taste even though he is an amazing player and going back to Manchester city seems to be the best option and choice if you ask me because he understands Manchester city teammates more than Barcelona and if we compare his appearance, that of Manchester city is more than that of Barcelona though he has not stayed too long in Barcelona.
Of course Pep Guardiola really needs a player like Gundogan to keep the midfield solid after Rodri's injury and De Bruyne's departure. In the match against Al Ain in the inter-club cup, he has shown that he is still worthy of being in the Man City first team for next season. Although he is no longer young, he showed calmness and of course that is very much needed by Pep Guardiola. In addition, Gundogan also admitted that he is very comfortable at Man City and he is still able to compete at the highest level and is ready to show his best performance every time given the opportunity by Pep Guardiola.
After moving to Barcelona a few years ago, Gundogan's performance was not as good as when he was at Man City. And that is also what made him return to Man City, one of the main factors is because Pep Guardiola was able to place him well and was able to motivate each player so that his performance was good again.