GPO can be backed up either by powershell or GUI
The cmdlet for backing up GPO is Backup-GPO -All -Path <path to GPO backup>
To backup via the GUI in GPMC right click on the Group Policy Objects container and select Back Up All. To backup a specific GPO, right click on the GPO and select backup.
Backing up a GPO does not backup attributes like security filtering, delegation, links, and WMI filtering.
To manage the backups, right click on the Group Policy Objects container and select Manage Backups, then browse to the folder where the backup are.
In the manage backup dialog box you can view the settings, restore or delete a backup.