It basically just means porn addict that spends far too much time masturbating non stop.

In the past we called them masturbation addicts as an affliction that had some negativity surrounding it and as something that people were considered victims of that even needed treatment of a kind. I wouldn’t be surprised if the porn industry had a hand in boosting “gooning” into collective consciousness as a shameless phrase that its participants hold no negativity for as a replacement for the previous.