Rumor has it that Galatasaray are very much interested in signing Donnarumma this summer, but I believe since Manchester City have also shown interest in Donnarumma this season, Donnarumma will prefer a move to Manchester City over Galatasary. Ederson will not accept to be a second choice goalkeeper if Manchester City signs Donnarumma, so it is expected Manchester City will sell Ederson to another club.
Some popular sports news channels reported the opposite of your comments. It has been reported that Gianluigi Donnarumma will be moving to Manchester City. Personal terms have been agreed, and City might be giving €30 million as a transfer fee. Meanwhile, 31-year-old Ederson has expressed his desire to move to the Turkish club Galatasaray. And the Turkish side has concluded with his earnings, but they are offering City about €15 million. Since the club already has good goalkeepers like Stefan Ortega and new signing James Trafford. So the Brazilian would have to leave. Within a few weeks, we might end up seeing these two deals materialise.