Gunn is from "Project Runway" and Harper is on "The Biggest Loser."
Ex-NFL star Eddie George and his wife, Taj, a former "Survivor" contestant, are also expected to serve as guest contributors.
"Each one of these contributors brings a unique perspective on achieving optimal health," Dr. Mehmet Oz said in a statement. "They have proven track records in inspiring others not only to realize their potential and self worth, but also to improve their lives. Body image, sports discipline and fitness are all incorporated into the show to help viewers fulfill their personal best."
The series, which Sony described as "an entertaining, high-octane, multi-topic hour hosted by Dr. Oz, vice-chair of the Department of Surgery and Professor of Surgery at Columbia University," is slated to debut Sept. 14.