The Big Dictionary of Dreams

by Martha Clarke
If you are gagged in the mouth, the dream manifests your inability to express yourself as you wish about a particular subject. Similarly, there may be something important that you want to say but do not know how. However, the meaning of this dream may be the opposite. Your unconscious could be alerting you to remain silent. (See MOUTH)

The dreams in which an obstacle that hinders communication appears often presage misunderstandings. You must be wary of rumors.

Dream Meanings of Versatile

by Versatile - Anonymous
Psychological / emotional perspective: Many people have difficulty in expressing their emotions, so a gag in dreams may symbolize the underlying emotional difficulty that prevents us from doing so. This might, for instance, be childhood trauma or the feeling that we will not be listened to.

To gag, as in experiencing nausea, suggests we are trying to get rid of negative feelings.

The Complete Guide to Interpreting Your Dreams

by Stearn Robinson - Tom Corbett
This follows the pattern of most obstacle dreams; if you got rid of the gag or managed to speak in spite of it, you will overcome your difficulties.

If your dream involved gagging someone else, or others who were gagged, it is a warning that if you are not more discreet, you may have to defend yourself against damaging gossip.