For me personally, gonzo is genre mixing + humor.
That humor could be tongue-in-cheek or camp or trolling. Breaking the 4th wall (or some kind of wall) is also a well-employed element of gonzo.
That's a great point.
The author talks about mixing science, fantasy and horror, but that is not a new thing, nor it is "gonzo" by itself. It is closer to the "weird" classics (CAS, HPL, REH) or the "new weird" (say, Ted Chiang, some GRRM stuff).
Even the (very derivative) Terry Brooks has his elves live in a post-apoc setting IIRC.
I do agree that "science fantasy" is not a good term for Wizards and Heavy Metal... not sure I'd call them "gonzo". But talking about RPGs here...
Venger's stuff definitely is.
Carcosa? Not so sure. It takes itself quite seriously (for some, too seriously).
But the interesting bit is that ORIGINAL D&D is very gonzo. It has humor, it mixes fantasy with science and horror, and it uses the "weird" authors I mentioned as inspiration.
The "boring Tolkien pastiche" is not what D&D originally was, that came later.
(And got old fast... which is why they came up with Dark Sun, etc.)
EDIT: so, gonzo (in RPGs) might be defined as "weird fiction + humor".
About weird fiction:
https://en.wikipedia.org/wiki/Weird_fiction