For those with issues I worked out how to get from the first GM to final Mavericks direct from the App Store without a second Mac/friend/etc.
1. Launch the App Store app
2. While its running, edit /System/Library/CoreServices/SystemVersion.plist
3. Change the version numbers to 10.8 for both and save.
4. Switch back to App Store, click on the big Mavericks Banner and it should say Free Upgrade rather than "Install". Click on it and itll ask for your Apple ID and associate it to your account. Now it should show up in your Purchase History, etc.
5. Now you've "bought" it, you can change your version numbers back to 10.9. Quit and relaunch App Store.
Make sure you launch the App Store first though. If you change your version number first it'll refuse to launch.