GM is an unusually gifted trainer and coach - if you get along with him. In the 80’s, it seemed that he taught every US international rider and a great many of the Big Eq finalists. He has definitely mellowed over the years. He used to be really only appropriate for and tolerant of higher level riders - it always seemed that if there was a 3’ section in a clinic, that those people were sitting ducks and he was very harsh. Today he seems to be more willing to work with riders who are below the level of the Big Eq.
I think he has always been someone who had to really think about his riding and be smart, rather than a seat of the pants natural rider. He is extremely detail oriented. That is why he is such a valuable coach.
Sometimes the details that are important to him aren’t so obviously important to other people. You want to go to a clinic turned out in a very specific way - not just clean and tidy, but impeccable in George fashion. Certain kinds of tack, certain kinds of clothes, and you will be unable to do anything right for him that day. I admit, this is why I love the scene in The Horse With The Flying Tail where GM is wearing an orange shirt with rust breeches… apparel that he would not tolerate in one of his clinics today. 
There are stories of him being both blunt/honest and also cruel. There’s a story of a girl in a pink sweater that comes to mind as an example of cruelty. But I haven’t heard any stories like that in the last decade, probably longer.
I would still say, though, that if you’re not at the level of the big Eq, you’d probably get as much or more out of auditing him and then doing similar exercises at home with your regular instructor rather than riding.