James will complete his transfer from Swansea City after this round of Euro qualifiers with Wales.
Wales teammate Gunter said: "I think one of the biggest strengths he has is his character and temperament.
"Now he's moving to such a big club, his life changes in terms of being in the spotlight more and things he has to deal with.
"He's very grounded and he'll take everything in his stride. I don't think he'll be changing too much as a person."
He continued: "If you get too close to him he can spin in behind. If you give him the space and try to protect that then he can pick the ball up and cause damage running at you.
"He's learning how to mix his game up more and different tests he's facing. His improvement and development has been phenomenal, really.
"Making the move that will continue to grow. As a team-mate that is a good thing, and not having to face him in the Championship."