I'm gonna say 16 points, with around 5 rebounds, 4 assists, and 1 steal per game.
My bold prediction is that he will have his best TS% of his career, eclipsing 60% (he was at 57.5 last year and his career high is 59.5).
16 points sounds like a pessimistic prediction, but he was at 11.5 last year, and his career average, if you take out last year, his injury year, and his rookie year, is only 17.17 points per game.
Gordon is a team-first guy whose instincts are to keep the ball moving and find open teammates. He's on a team with lots of scoring talent -- Kemba, Tatum, Kanter, Brown, not to mention some talented young guys on the bench.
To score 20+ points on this team Gordon would have to dominate the ball and look for his own shot in a way that I just don't think is likely.