So if I'm currently on 9.1 with no syncs, and no backups other than 9.0 GM from a few days ago ...the method is to download final 9.0 ipsw, option + restore to 9.0 GM.
I didn't need backups for my downgrade from 9.1 beta to 9.0 official. Just shift click (Windows), select the file and upgrade. It restored with all my apps and settings intact. iPhone 5s and iPad Air 2.
I did full backups AFTER I did the restores. Make sure to get rid of the beta profiles before you do this.
This is where I got my official Apple ipsw files:
http://www.redmondpie.com/ios-9-download-links-for-iphone-ipad-ipod-touch-ipsw-links/