I don't think it will be wise for Donnarumma to join Galatasaray when he has an option like Manchester City, because it will be a down grade from my own perspective. He is a world glass goal keeper a day deserves to be playing in the big leagues and I also believe that Manchester City will be a good fit for him.
Ederson is also a very good goal keeper and Galatasaray could make do of that option and it will also be a good bargain for them.
What if he has weighed his options and see that it will be harder for him in the English Premier League that the Turkish league. Sometimes these players do weigh their options and knows where they fit in. I am not saying he can't be a goal keeper in the premier league or he can't do well there. The Turkish league is not bad either, it's not every player that the premier league favours.