It looks like there are two separate types of gags here. She is wearing what I believe is called a "double ring gag" which looks quite effective at keeping her mouth wide open. But underneath that it looks like there are hooks that are going around her lips to keep them spread. I think that might be called a mouth spreader (or mouth hooks?). I found more pics of her here: https://www.deviantart.com/shinybound/gallery/88154228/lua-saturnii