Maybe that's what the player wanted. I think this decision was made in a centralised way, the management discussed it with the player himself and stood on his side without interfering. In fact, if it really was like that, it is a very favourable departure from the club. What's more, he will be no worse off at barcelona, staying at the same level
Apart from being very profitable for the club, Gundogan himself also has nothing to lose by leaving for a big club like Barcelona because he didn't go to a bad club. Gundogan can still have a very good career at Barcelona at a level that is not much different from before so that Barcelona will also benefit enough to have Gundogan next season. Because that will make Barcelona stronger to compete with other English teams when they meet in the UCL competition.
As of right now I would say that there is at least a one level difference between Manchester City and Barcelona. I can still understand that he transferred to the Spanish club if he wants to enjoy life over there while fulfilling one of his biggest dreams, but he certainly isn't joining a team that is of the same quality as Manchester City. Maybe in three years from now and even that is unrealistic.
I am also not sure whether he is making the team stronger when they are also planning to sell de Jong or exchange him for another player. It is not the progress that I thought they are looking for to gear up for the Champions League, but we will see what they are up to in the coming weeks.