I'm pretty sure the people who came up with the term struggle with the definition as well. But like a lot of fiction terms, its one of those things where its usefulness begins and ends with describing a piece of fiction. "Oh, that's gonzo. What's gonzo? I don't know, but that's gonzo."
Not that I like that approach. I think its very... buzz wordy, and I don't care for buzz words. That's why I had to try to figure out a definition that at least was useful for myself. But I probably wouldn't have written this if I hadn't seen Noctis Labyrinth described as gonzo / haven't had sessions that took NL to the ridiculous over the top play that can be described as gonzo.