if you want to do a wmi filter in the GPMC for apply a specific gpo only to a specific operating system follow these steps:
In the GPMC go to WMI Filters and click New.
Writte a Name and description and click Add
Writte the following sentence:
select * from Win32_OperatingSystem WHERE Version like “6.1%” AND ProductType=”1″
Now you can apply this wmi filtering to gpos.