"Wanking" and "Gooning" have vastly different meanings, in their context and nuance.
Gooning is really about the visual stimulation, the content being viewed. You goon at or to something.
Wanking on the other hand (or with the same hand perhaps) is purely about the physical act of grabbing your cock and jerking it.
Besides that, there are more subtle differences. Gooning implies prolonged and leisurely, for example, while wanking does not.
You'd never say you had a "quick goon," but you could certainly say you had a "quick wank."
So don't worry, wanking isn't dying out any time soon.
EDIT: Added a handy reference table
| Activity Type | Visual Stimulation | Duration | Gender Neutral | |
|---|---|---|---|---|
| Gooning | Mental | Required (Typically porn) | Long | Yes |
| Wanking | Physical | Optional | Long OR Short | No (Requires penis) |
