I think I tend to see Goku more as a force of nature. I can't quite remember what Gaiden says, but I'm pretty sure it does use the term "born," so while it's out of rock, it's still situated on the natural side of things.

Or I'm remembering wrong and that's Monkey from Journey to the West.

Anyhoo, it feels like while there's this great emphasis on Nataku being created by someone else, Goku sort of springs into life full-formed.