Jeff Hicks, one of Microsoft’s Powershell MVPs, has written a great script for managing GPO backups:
The GroupPolicy module from Microsoft offers a great deal of functionality from a command line. In terms of regular maintenance or administration it is pretty hard to beat, especially if you have 100s or 1000s of GPOs. When you have such a large number, backing them up is critical and easy to accomplish with the Backup-GPO cmdlet. However, if you want to restore or import from a backup, you have to resort to the graphical Group Policy Managment Console. I couldn’t find any way to manage GPO backups from PowerShell so I wrote my own.
It also has explanations for how the script functions, and how to use it. The download link is at the bottom of his post.