A penis gag is a gag consisting of a (usually rubber) penis or dildo (typically only a few cm long) which is attached to a leather plate (to prevent it being swallowed) and forced into the victim's mouth.
Particular care must be taken when playing with penis gags; because of their size, they tend to get pushed far back into the mouth. This means that there is a significant risk of the wearer choking. Some in the BDSM community suggest that penis gags are better as an idea than in practice. They can move around the mouth much more than a ball gag and that makes them less effective in gagging the victim and making it easier for him/her to spit the gag out. They can be made more effective by using them in conjunction with a hood or a ring gag.
There are also gags that feature a dildo/penis attachment on the outside of the face plate, so that the victim can be used by play-partners as a very literal sex toy. Care should be used in this type of scene–broken teeth or facial injuries can halt a scene in a very distressing way.