Gooning is a more specific subtype of masturbation.

Masturbating is the act of physically stimulating one’s happy parts, solo. It may involve porn, or it may just depend purely on the physical sensation to lead to orgasm.

Gooning has a much closer association with visual stimulation because one would generally goon at something or to something. It also has a strong implication that it usually lasts for a long while; that the point of gooning is to stretch out the pleasure as much as possible and to push away all thoughts until you are nothing but a drooling mess of ecstacy.

So it really is quite different.

I made a helpful table