You do realise that there is no "American" version and "International" version of the 3GS? Nor was there an "AT&T GSM" version and a "CDMA" one.
I realize that this is an international community.
"America", not "American", and "AT&T" refer to the users, consumers, current buyers, etc. not the device. AT&T currently sell the 3GS. If iOS 6 is not available to those purchasers, then those purchasers will not be able to upgrade their software. A common stated benefit of purchasing an iPhone is Apple updates the software, unlike other phone manufactures. So, those purchasers are not retaining that benefit. Unfortunate fact (maybe, don't know yet).
I am not knowledgeable about other nations cellular sales plans.
I do not know if they sell 3GS. I remember the 3GS being listed #3 or #5 most popular/sold smartphone. I do know some cellular service providers have sold the iPhone 4 free with the long-term contract.
I see you are from Melbourne, Australia. One of my favorite places, very beautiful. Could you give any details about the 3GS sales of Australia?
As far as my description of the phone as GSM, yes, overly descriptive. You have me there.

I hope I have clarified statements better now.
Thank you for your comments.