I haven't heard that Galatasaray has plans to sign Donnarumma. From the little information that I got, it's not certain that Donnarumma will move to Galatasaray despite that Galatasaray plan to give him a three years contract with a transfer fee of €10 million. For Donnarumma to be in City, City needs to sell Ederson in order for them to use the money to sign Donnarumma but if Donnarumma chooses Galatasaray, Ederson remains in City.
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.
You are right, it has been rumoured that Donnarumma is being sought after by Galatasary, but it was also reported that Donnarumma has agreed personal terms with Manchester city and I also read from somewhere that Manchester city and PSG have agreed a deal worth £35 million pounds for Donnarumma but they will only push to finalise the deal, if they are able to sell Ederson to another club. It was Galatasary that was said to be the front runner for Ederson signature but I don't know if they are still interested to sign him. Am sure Donnarumma will prefer a move to Manchester city than going to Galatasary.