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Manchester City has their policy to allow players to leave their clubs if those players submit a request and show that they are no longer want to contribute for the club. Gundogan is not an exception and his age is another factor to help him leaving from Manchester City.
you are right that Man City might allow Gündogan to leave City because he is indeed quite old. so that Man City has the goal of releasing Gündogan and trying to find young players who have better talent to be able to bring Man City to the defending champion in the champions league.
I know maybe Guardiola has another new dream to keep winning the treble again in the new season so he sells several players who are not performing well to welcome next summer.