There are two different mechanisms inside GPMC that can be used to restore a GPO.

The first is a simple right-click of the GPO that you want to restore, and choosing the option called Restore from Backup....

This invokes the Restore Group Policy Object Wizard, which is a pretty fast and simple walk-through. Specify the location of your backup files (if you always back up to the same folder, it auto-inserts this information), and then the wizard will display all of the different versions inside this backup folder that exist for this specific GPO. You can see in the following screenshot that I have selected to restore my Auto-launch applications GPO, and there are three different rollback points listed. You can easily determine which is the newest by the Time Stamp displayed for each backup copy, as well as being able to review the notes that I typed when I was performing these backups. You can even use the View Settings... button to open up the HTML report of what is inside this GPO. That is an awesome feature:

After selecting the version of backup that you want to restore, click Next and Finish and the GPO is immediately rolled back to the previous settings.