1) The abreviation of "Gaping open mouth, silent and gesturing"
2) The action of gesturing towards a truley shocking or deviant scene with both hands (fully extended, palms upwards) and mouth (and possibley eyes) wide open while in a state of stunned silence.
*They action may be recorded as <GOMSAG>
Chad : hey, tony, wanna come do cock-push-ups with us?
Tony : <GOMSAG>