If the public release has a different version number than GM you'll have to download it from a direct link to Apple (or wherever else you find it) because neither iTunes nor OTA will pick it up as an update. I think this is how it's been in the past. Basically the way I understand it, the GM is the first version of iOS 7 out of beta, and in the past it wasn't possible to upgrade from beta to GM OTA. Had to be done via iTunes after downloading the ipsw.
What I'll do is download the GM and update the software on top of the beta and then when the public launch occurs I will do a fresh install.
Oh well...I was hoping when GM came out I can jump straight to it from iOS 6 but since there a possibility of the public being a different version and me having to do a fresh install...seems like Im better off waiting for the Public release.