You really shouldn't be adding fonts to that directory directly. Fonts need to be installed on the system. That involves more than just copying files. You should install the fonts onto the machine using the normal process. This eliminates your issues. To do that you would create a logon script. One example of this is provided here.
If you really, really need to use GPO to do that then you shouldn't need to change the permissions on the folder. More specifically you should never change the permissions on the folder using GPO and then fail to set the permissions back. If you do then you are potentially exposing the machines to malicious use as other users can now modify that directory as well. To test locally you would need to elevate your permissions when copying the files but this shouldn't be an issue if you're using GPO as it is already running at the appropriate permissions. But, again, I would recommend you use a logon script instead. Let the font install itself.