What I would do is go to the link listed below and download the policy definitions installer, extract them to your desktop.
I would then copy the new policy definitions folder and paste it in the C:|windows file structure.
By default you can not add any files to this folder and subfolder, you need to take ownership and add the modify right to the admin group.
the folder, subfolder, and all files should replace the previous files.
then open GPMC and it should work.
If you open GPMC look at the administrative templates and it will state if it is local or central store.
You can also read this blog to help you out:
https://techcommunity.microsoft.com/t5/itops-talk-blog/step-by-step-managing-windows-10-with-administrative-admx/ba-p/842926