If she is walking well, loose leash on your walks then I dont see why you would need the GL, as long as you keep up on the loose leash training when walking.
I'm assuming the trainer reccomended the GL because she is pulling more in class and around people more then when regularly walking, is that right?
The GL should be used when she is excited, when you percieve she is going to act up and should be used with the same command you give when loose leash walking so she knows what is expected of her. She'll soon get the idea that she wears the GL because she cant contain her excitement in class with other dogs or in busy places.
Rory was trained with the GL, she now walks very well but I keep it in the car incase we go to a heavgy populated area like a festival or farmers market. Actually now all I have to do is take oit out and threaten her with it and she smarts up real fast, he he!