Gundogan is a well-appreciated and decorated player and Pep Guardiola always speaks highly of him. As a free agent from the end of this month, any club that buys him will surely enjoy the benefits but I do not see a year contract as the issue here.
He has performed admirably for Manchester City, and I agree with you that he still possesses the qualities of a good player despite his advanced age. However, I believe that living Manchester City would be a kind of downgrade in my opinion, given how many players now consider the team to be their favorite with their performance these season. As for the salary issue, I don't take it seriously because if they weren't paying him well, he could have maintained his position for all these while, I only think maybe his contract has come to the end, and they are not willing to renew it for him to continue playing for them which is why I think they want to transfer him to another team.
However I just hope Gundogan will choose good team with well experience players, since I can see many teams are looking for him.