Gobbler yelping is a very deadly call, we use it alot in the fall, and i've called alot of lone gobblers in during the spring as well. I do however think that it is the one call that makes a guy sound like a "guy" instead of a turkey,...if not practiced and presented right. It's not just as simple as slowing down your regular yelp or drawing it out more, or making it deeper. In our competitions, there are only a handful of guys who can nail a true gobbler yelp,...it's just that hard. Chris Parrish, Brian Gulvas, Denny Gulvas are some of the best i've ever heard. It must have a beginning, middle and end, like your regular hen yelp, but the tone, and rythm are a sound all their own. I guess what i'm saying is, don't settle for just sounding like a guy,...listen to some real birds and go with that. I have one slate call that gets "close" but what works best for me is a 5 reed mouth call in a ghost cut. good luck.
Pat