The Gentle Leader is one of the tools to help making your training easier for a problem dog. You still train your dog while using the tool.
Once you and your dog have mastered
healing etc. you can do away with the Gentle
leader if you wish, or keep on using it.

A method that I learned to help get your dog use to the G.L.
You can use a clicker or praise and treats.
Have a treat handy in your hand by
your chest, so you get your dogs attention while you are walking.
After you walk a few feet and if your dog
is healing and eye contact between you
and your dog has been met,
use the clicker or voice and give a treat/praise.
If your dog is pulling ahead than you just stop walking. (no treat or praise)
Same method as others have mentioned but
you are using tools.
By using this method it keeps the dog
mind off the G.L. and more
on you. The dog also learns that if he
forges ahead of you, he does not get a treat/or praise.
Another suggestion for those using a G.L.
Have a choke chain hooked onto the leash with the G.L. That way
if your dog breaks or slips out of it
you have the choke chain on for back up.

My problem is with aggressive dogs.
I could not walk my young still growing
100lb. aggressive male past other
aggressive dogs.
After the first week of using the Gentle Leader with the clicker/treats method. I am now able to walk with just using the Gentle-Leader. He will cry when we are passing
the aggressive dog but he looks at me
instead of lunging.
Try what ever method works for you and your dog. No way is wrong or right just
so it is humane.