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.