The settings of a backed up GPO can be imported into an existing GPO. To do this via powershell the cmdlet is:
import-gpo -BackupGpoName TestGPO -TargetName TestGPO -path c:\backups
In the GPMC, right click on the GPO where the settings will be imported into and click Import Settings.
The settings that are imported into the GPO will overwrite any existing settings in the GPO.