A Global Catalog server stores a copy of all the objects in Active Directory, this allows us to save network bandwidth between sites.
Another reason to have more than one GC is in case the main global catalog server goes offline, users will still be able to log on to their computer by being authenticated by the backup GC.
If the only GC in the domain goes offline and Group Membership Caching is not enable on a secondary domain controller users will not be able to log on to their computers.
This article will show you how to enable or disable GC on a domain controller, remember that its recommended to have at least on GC on each site.
To enable or Disable GC on a domain controller follow the steps below.
Open Active Directory Site And Services -> Go to the specific site -> Click on Servers
Click on the server -> Right Click on the NTDS Settings in the left panel
Click on Properties.
Here you can enable or disable the server from acting as a Global Catalog server.