In bondage; device used to prevent the submissive from talking. There are lots of different types of gag device on the BDSM market. It can be made out of clothes, rubber and leather.
Gowords:
Like a safe word, a "Go Word" is an OK from the Submissive to go ahead with the play, when there was a stop to a play (usually after a "Safeword" or a "Slow word")
We talk about the Goword in our article at: Safewords.