(I no longer need to use my gentle leader.)
When I did, and my dog was first getting
use to it, I would carry around treats
in my hand. (I would not allow him to paw at it or rub his nose on the ground)
When he walked nicely I would praise him giving him treats.
Depending on the dog it can take awhile
getting use to it. My female dog with the leader was a pain, but my male, with a little patience and treats got use to it fairly fast.