I believe SWHouston is right about Gizmo's motivation. Reminds me of a few instances when Max was younger. He still thinks he is protecting me from danger on our walks (that is what our vet and many other people have told me). When I held him back and tried to quiet him when a big dog passed, a dog we could not meet because it wasn't safe, he went into attack mode and turned and nipped me for trying to hold him back.
Thankfully, Max has great bite inhibition. He can use his teeth without doing harm. Not using teeth at all is ideal, but it is crucial that a dog have bite inhibition:
http://www.yorkietalk.com/forums/gen...nhibition.html