I have found the answer to this question.
By default, the GP Editor only shows settings that can be fully managed by Group Policy (those that can be set, and that the user cannot change), as opposed to ones those can be partially managed (where GP can set them, but the user can then edit them).
To show both types of policies, follow these steps -
- In the GP Editor, first left click (so that it is in focus) and then right click 'Administrative Templates' under 'User Configuration'.
- Click 'View -> Filtering...'
- Uncheck 'Only show policy settings that can be fully managed'
- Click 'OK', hit 'F5' to refresh the GP Editor, and go look for your policy.
Also see this article on TechNet - http://technet.microsoft.com/en-us/library/cc739211%28v=ws.10%29.aspx