I don't know Gordon, an excellent fighter in his prime but can we call him an ATG? Such a small length of time at the top with no massive names, Apart from an aged Holmes. He's one of my favorites but I'm a little unclear about calling him ATG. Perhaps I could put him at number 10.
I think this is a good number: 1. Ali 2. Louis 3. Holmes 4. Holyfield 5. Lewis 6. Foreman 7. Marciano 8. Tyson 9. Frazier 10. Wlad 11. Dempsey 12. Johnson 13. Liston 14. Vitali 15. Charles 16. Bowe 17. Tunney 18. Walcott 19. Fitzsimmons 20. Norton
I'll define "all time great" as an elite circle in which any fighter within it has a nontrivial chance of beating any other (with maybe one or two exceptions of a bad style mismatch). It's very difficult to say how many that is but I'd guess anywhere from 20 to 50.
He's definitely an ATG, just not at heavyweight (in my view). Charles is borderline. The only other guy I'd consider as an ATG heavyweight is Liston but his title reign plays against it.
By definition zero, how can you be an ALL-Time great? But not taking things literally, purely for 'Heavyweight work' perhaps only Ali and Louis. 'Iconic status' probably pushes a few more towards greatness (Jeffries, Johnson, Dempsey, Marciano, Foreman, Holmes, Holyfield and Lewis), and of course as part of an overall resume, Messrs Fitzsimmons, Tunney, Charles and Moore are in there.