''Gooning'' may be most simply defined as that state usually achieved after a prolonged edging session, when a man becomes completely hypnotized by the beautiful sight of his own erection, so that he no longer really needs other stimulation such as pornographic words and imagery. He's turned on by witnessing his own sexual arousal -- that is, he's turned on by the fact of his turned-on-ness -- and by simply gazing at his own hard, throbbing, precum-leaking, magnificently MALE PENIS.
He may also have a sensation that he is his cock, and his cock is him, and furthermore, that nothing exists except himself and the DONG. (Being a geek, I once suggested that the goon state could be compared to that episode of Star Trek: The Next Generation where a subspace fluctuation in the warp drive's matter/antimatter ganeckticazoid ends up flinging Beverly Crusher into a collapsing micro-universe that contains only herself and an unmanned U.S.S. Enterprise.) Anyway, while all this is happening, there's a high probability that his face is contorted into some mighty undignified expressions -- and hence the term "goon."
Lil Bro: I'm not JUST jacking off bro, I'm fucking gooning on this dick!
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).
Check out gooning.org