The last beta of iOS 5 which was release of Beta 07 which has now been followed by the GM release. The GM comes just days before Apple releases the final version of iOS 5 which will be released on October 12.
October 12, will also see the release of Apple cloud based service - iCloud alongside iOS 5. The iPhone 4S will be released on 2 days later on October 14.
There should be no changes between the GM and final release, but you have to be a registered Apple developer in order to get the update. If you are a registered developer you can download iOS 5 GM from here.