These are a popular style for bondage/BDSM purposes as they leave the wearer vulnerable to forced oral sex, and anything else can be pushed or poured into the mouth against their will.
They are most commonly made from metal (sometimes with a covering or coating) and they include the Jennings gag, the Whitehead gag, ring gag, spider gag, and the tube gag. The first two are adjustable medical gags that will particularly suit doctor/nurse fetish scenes. The funnel gag can also be considered a mouth opening gag as it allows forced drinking of fluids.
They generally fit between the teeth and are held in place by a strap fastened behind the head. More intricate designs come with a harness that makes it less likely to slip out of the mouth. The strap or harness can be made from leather, cloth, rubber or even silk in some cases, and either fastens with buckles or some versions are able to be tied.