Pep Guardiola has shown in the past that he's very good in replacing members of his squad immediately they leave his club and he does it so efficiently that the player's absence wouldn't be felt no matter how important he was to the team's progress. He did it after the departure of Vincent Kompany, Yaya Toure, Sergio Aguero, and David Silva .
However I think IIkay Gundogan still has a lot in the tank to give out when it comes to delivering top performances for ManCity but time will tell if he'll remain in the club to do more for them or be allowed to leave
Pep Guardiola managed to overhaul his players after being abandoned by some of his veteran players and this made Manchester City even more feared for any team that became his opponent. If Pep Guardiola can maintain his current squad for the next few seasons I think they will continue to dominate the EPL or even Europe.
Regarding Gundogan who will leave Manchester City, I think he will still consider his next career because after winning the treble winner for the first time for Manchester City, sometimes players feel satisfied and want to leave the club immediately to start a new challenge, but if Gundogan wants to stay then he will still be able to win everything if with manchester city.