I'm going to give you my opinion but I'm no expert and I would rather on of the trainers gave you the real scoop.
When I took my energetic 4 month old pup to puppy classes she was dubbed "the wild child" and the trainer suggested that I put her in a Gentle Leader. The trainer got the collar for us and I learned something immediately. The collar MUST be properly fitted in order to get maximum benefit.
My Casey turns into what my mother terms as " A Humble Bumble" when the collar is put on her. You can see her spirit actually deflate! At first, the trainer told me its just because she's not used to having something across her nose but no matter how long its on, she doesn't bounce back. But, in my pup's case, I have complete control over her while she has it on. I don't actually ever have to pull. But as soon as its off she's right back to her old self....including the not listening parts! So, to me, the collar is just a method of control.
Volunteering at a shelter is admirable and worthwhile work. Those dogs need your support. Perhaps the shelter could explain to you why they want the Gentle Leaders used. Perhaps they are using them for safety reasons or because some dogs have been mistreated and are sensitive to collars around the neck (I've seen dogs like that!) In return you could express your concerns. Find out if the staff do the fittings or teach you how to do it. You could try suggesting the Martingale and see how receptive they are to you using it. At the end of the day though, you really are there to walk the dogs....not train them, right? And interaction with you and the walking is good for them in my opinion.
