Running gpmgnt.msc will give you a local editor. Instead run Group Policy Management on the Domain Controller itself
Nagivate to Group Policy Objects from Forests / Domains / domain.local
Right click the policy / or create new to open a copy of the group policy editor that is connected to the domain. You will know this as it will show [DOMAIN.LOCAL] Policy in the policy title.
If you right click a policy, under GPO Status you can set the permission to change it for clients. e.g User Configurations Settings disabled Will enforce the domain policy and stop users from being able to customise it.
