Whitehead gag
From wipipedia.org
A Whitehead gag, also called a mouth spreader gag or a dental open mouth gag, is a device to hold the mouth open. The device is similar to a Jennings gag the main difference being the two rachets in front of the gag to hold the mouth open. The image shows a whitehead gag modified for BDSM use with a strap added to help hold the gag in place.
For issues concerning the use of this type of gag read the article on the Jennings gag.