If you do a little more research, you will find some magazine articles talking about the program. They will only disclose the associated courses to prospective members and it seems that even the journalists reviewing the program had to sign nondisclosure agreements because none of them disclosed the courses. One article did say that there were some big names on the list, but then said that "no, Baltusrol is not one of them". While the program may have expanded, one article did imply that, if you don't play in the NYC tri-state area (NY/NJ/CT), you will have a tough time using the program.

From what I have been able to glean, there are possibly some screening criteria as to who they "let" into the program - the clubs probably have demanded some standards for the people who will play on their courses. Although I hazard a guess that anyone willing to pay $3,000+ for 5 rounds of golf will find it pretty easy to get in the program.

I did also read that, once you have a membership, when you want to play a course, Tour GCX will set up the time and then provide you contact information to someone at the course. You contact the person who meets you upon arrival and takes you to the locker room where you have a locker with your own name plate on it. You pretty much have full access to the club as if you were a member, including dining privileges, etc.

Since you've been to the site, you probably saw that this program is not limited to Amex - it is available to others too. It's just that Amex lets you pay around $3,000 for 5 rounds of golf with a foursome, while others have to pay over $6,000 for 10 rounds of golf. It is exactly twice as much for twice the golf - so Amex does allow a little less of a commitment to the program.

Google Tour GCX and I'm sure you'll find things.