There are short descriptions of his videos at midwestturkeycall.com Look in the book/video section. Heres a short bio from the realtree pro staff page...
Denny Gulvas of DuBois, Pennsylvania, is one of Realtree's original pro staffers. Having followed the Professional Turkey Calling Circuit throughout the country for many years, Denny has won all the major titles including the Grand Nationals. He's a 3-time Master Invitational Champion, a 4-time U.S. Open Champion and the Pennsylvania State Champion, not to mention a host of other state and regional titles.
A wildlife video producer, Denny has spent more than 37 years studying and filming all aspects of the wild turkey with a specialization in the vocal communications of this grand game bird. As a major turkey call manufacturer, the unique Gulvas Diaphragm Mouth Calls have been successfully used throughout the country both in the field and on the competitive stages. As a highly regarded authority on the hunting and voice of the wild turkey, Denny presents exciting and unique video-assisted seminars throughout the country helping today's turkey hunters better understand this very challenging sport.