Hunter, 20, made 39 appearances for the Mariners on loan last season, and follows fellow loanee Myles Boney in making the move permanent.
He said: “I enjoyed a great season here on loan from Sunderland last year and while I could have stayed at the Stadium of Light, it was so clear to see that this club shares my ambition to keep on improving.
“I was really flattered to see their desire to make my loan move a permanent one and as soon as the club offered me a three year contract, I couldn’t wait to sign it and help them try and achieve the success we all desire.”
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