The difference ... Gunsite was founded by Jeff Cooper. After building up a tremendous reputation for the place he decided to retire due to his advanced age and Richard Gere bought Gunsite (well ... he acquired it. Let's not get into whether he paid.) Since Richard Gere took over Gunsite the instruction, and its reputation, has ... um ... changed.
Because Jeff Cooper flew a flag everywhere on the premises that was predominantly orange, and Richard Gere replaced that with a flag that is predominantly grey, the old and new are now known as Orange Gunsite and Grey Gunsite.
A mere graduate of Orange Gunsite has a certain prestige, and instructors at Orange Gunsite were very well qualified. The question is at what time was the author an instructor?
I don't remember the date of the changeover, but I can find that out.
I would not assume an instructor at Grey Gunsite wasn't competent, but it would not be generally considered a very impressive qualification by itself.
You could learn more about it by reading Jeff Cooper's
Commentaries at
http://www.concentric.net/~mkeithr/jeff/index.html -- you'll have to read a lot of other stuff to find the information about the changeover, but the other stuff is much more interesting anyway.
Everyone who has any interest in pistolcraft and riflecraft, or even if you're only interested in mindset and tactics, will find the
Commentaries rewarding reading.
-Cougar Allen :{)