A gag in the shape of a ring, used in BDSM activities. A ring gag prevents speech, because it
forces the mouth open, but it does not prevent the
wearer from making GAAARGH - AWG sounds.
The point of the ring gag is that the submissive
wearer's mouth and tongue are, to a certain extent, available for whatever activities the dominant may
think fit.
Ring gags are, like nose hooks, quite frequent in Japanese porn.