Gundogan, 34, leaves City after making 358 appearances and scoring 65 goals across two spells with the club. He was Pep Guardiola's first signing in 2016, and during his time at the Etihad Stadium, he won five Premier League titles, the Champions League, and captained the team to a historic Treble.
After a year at Barcelona, Gundogan rejoined City in August 2024, making 33 Premier League appearances last season as City finished third. However, he did not feature in City's first three games of the current season and was left out of the squad for the 2-0 defeat to Tottenham.
Reflecting on his time at City, Gundogan said:
"Manchester City will always hold a special place in my heart. We have enjoyed so much success in our time together and there have been so many unforgettable moments, not least being captain for the Treble-winning season.
"To win the Premier League and FA Cup trophies meant so much to me, but to then be afforded the opportunity of lifting the Champions League trophy for this club for the first time, especially in Istanbul, will live with me forever.
"I have no doubts Manchester City will enjoy much more success in the future and I will certainly be watching on from afar as I continue my career in Turkey, a country that means ever such a lot to me."
Gundogan, who was born in Germany to Turkish parents, previously played for Bochum, Nuremberg, and Borussia Dortmund - winning the Bundesliga title with Dortmund in 2012. He has been capped 82 times for Germany and will now play alongside former national teammate Leroy Sane at Galatasaray.