Since you only have one site, you only need one boundary/group for site assignment. Why do you prefer a GPO?
If you prefer using a GPO, you can try the ADM "ConfigMgrAssignment.adm" in the SMSSETUP\TOOLS\ConfigMgrADMTemplates folder of the install media.
SCCM 2007 documentation (not sure about 2012):
http://technet.microsoft.com/en-us/library/bb680370.aspx