It's a training tool in the sense that you should still be, well, training, loose-leash walking. Walk him on his regular collar in the house. Reward him heavily for staying by your side (or wherever it is that you like him to be on walks). When he's doing that well in the house, then you move to your backyard, your frontyard, the sidewalk in front of your house, etc. You slowly move up to longer walks, more distracting environments. Being next to you on a walk should be his favorite place to be. At first, maybe you need to reward him every time you take 1 step and he stays at your side. When that is reliable, then take 2 steps, 3 steps, etc. If he pulls, you need to stop walking EVERY time, and then gently pull him back toward you, continuing the walk only when he is at your side. It's important that you're consistent with this, otherwise he's learning that pulling gets him where he wants to go.
The thing with the GL is that, ideally, it should only be used when your dog is going to be walking somewhere that you know is going to be above their threshold. The GL doesn't actually do any of the training--all the hard work still needs to be put in, the GL just allows you to go for walks without reinforcing the pulling or having your arms pulled out, while you're still working on training your dog to walk nicely without it. Of course, some people would rather skip the training and just accept that their dog will always need a GL, and that's fine too.
Hope that makes some sense.