A panel gag is a type of gag. It consists of a panel, typically of leather, that covers the mouth and has a ball attached. The ball is held in the mouth by the panel, which is secured to the head by a head harness or straps. Panel gags without an attached ball are purely cosmetic, as they do little to restrict speech.
A muzzle gag looks similar but does not necessarily have an attached ball. However, it may restrict speech by immobilising the jaw.