"Gooning" may be most simply defined as that state usually achieved after a prolonged edging session, when a man becomes completely hypnotized by the feeling radiating his penis. Since a gooning state can only be achieved after edging, the man's dick will have become mightily aroused at this point and every caress the male genitals are subjected to will trigger potent elation. As the man keeps edging and thus keeps experiencing intense pleasure, he enters a state of trance where his mind intimately merges with his cock : the gooning state, where he and his dick become one. To be even more accurate, when the gooning state is achieved, the man's body becomes for all intents and purposes an appendage to his erection. When this state is achieved, the male becomes freed of all social codes of conduct, and his arousal, alone, dictates his reactions. As a result, a gooned out man will become very expressive and demonstrative : he may become very vocal, while his body and face might take on undignified expressions and poses, all in response to the intensely exquisite caresses his penis is exposed to. Hence the term "goon", since at this point the man effectively looks like a silly, foolish, or eccentric person.
Gooning is not a narcissistic manifestation. Narcissism is extreme selfishness, with a grandiose view of one's own talents. Rather, gooning is closer to a meditation experience, where the mind and the body align, focused on a single thought (or feeling, in this case).
I spent 10 hours gooning over the weekend.