It's a dude who goes on a jerk off marathon for hours at a time, and I'm sure there's a few that will do days. It can be any combination of edging or shooting one off as soon as he's reloaded. Wouldn't even necessarily have to be with porn, gooning is all about the peen.
Some say that a proper goon sesh is a transcendant meditative state, where the mind is no longer present and has merged all consciousness with the dong mind.
It seems to me gooning is actually fetishizing your porn addiction to the extreme. Like it gets you off that you are addicted to porn and will spend hours on end masturbating.