Pardon my ignorance, but I always thought the GM was the public release. What's the difference?
99 percent of the time it's the same, but if there is a major bug that needs to be fixed immediately, there will be a new build.
Ex. When Apple released the iOS 4.2 GM, there was a WiFi bug in it causing Apple to have to fix the bug and release a new GM (iOS 4.2.1).
http://bgr.com/2010/11/22/apple-releases-ios-4-2-update-for-the-ipad-iphone-and-ipod-touch/
The GM is released early so that developers can begin getting their apps ready and test them on the final version so they can be released on the App Store when releases the update to the public.