The Ghana international has been linked with Gala in recent weeks.
“We agreed on everything over the summer," he is quoted by TuttoJuve as telling the Turkish Press.
“After drawn-out negotiations, I made the best decision for myself. I gave my word to Galatasaray and I won't go back on it.
“I picked them for the role they gave me in their team and what they offered me.
“Now I'm thinking about Juve, but Galatasaray are important to me and I'm following all their games with great interest.
“Their football is promising and I look forward to becoming a part of this team."