v (intransitive) To experience the vomiting reflex.
v (rfc-sense)(context|U.S. Army|slang) To smoke#Transitive_verb | smoke: to order a recruit to exercise until he "gags" (usually spoken in exaggeration).
v (transitive) To restrain someone's speech by blocking his or her mouth.
v (transitive|figuratively) To restrain someone's speech without using physical means.