The GC is a service that provides an efficient way for clients to locate objects that lie beyond the local domain. The GC hosts a portion of each domain partition, enough for the client to locate domain objects not found in the local domain. This means that if the data located in the GC changes, the changes must be replicated to all the GC servers in Active Directory. GC servers are DCs that host not only the domain, schema, and configure partitions, but the GC partition as well.
As previously mentioned, each client must have access to a GC server for a logon access token to be created. T he placement of GC servers within the organization is therefore important. It is recommended that each site has at least one GC server. Those sites that are split between two or more physical locations might need a GC server at each physical location in case the WAN connection between the physical location fails. With a GC server at each site location , clients would still have access to it, even if a WAN connection failed.
Why not make all DCs GC servers? Doing so increases the amount of GC server replication and requires additional resources on all DCs. In order to reduce the amount of replication and resources that are required for DCs, strategically place GC servers throughout the organization so that all clients and servers have access to a GC server on the LAN.