I don't know about PetCo training; these days many PetsMart trainers are actually quite good. I advise observing a class or two - for any trainer - before you decide whether or not to use them; that way you have a better chance of avoiding nasty 'surprises', like harsh corrections. Plus you can see how the trainer addresses specific issues and individuals, and whether the students are getting the help they need. (When it comes down to it, training classes are meant to teach you to teach your dog - so the human partner needs to be getting information they can use!)
Haltis/Gentle Leaders are training tools, just like chokers or prongs, though they are not meant to be used for giving corrections. As with any tool, you still need to teach the dog the desired behavior, so that you don't have to rely on the Halti (or collar, or lure, or target) for the rest of the dog's life.