What's gc? how can you migrate gc from one dc to another?

To promote or Demote a domain controller to a global catalog server, follow these steps:

1. On the domain controller, click Start, point to Programs, click Administrative Tools, and then click Active Directory Sites and Services.
2. In the console tree, double-click Sites, double-click the name of the site, and then double-click Servers.
3. Double-click the target domain controller.
4. In the details pane, right-click NTDS Settings, and then click Properties.
5. On the General tab, click to select the Global catalog check box.
6. Restart the domain controller.

Just select or deselect the check box of Global Catalog as described at5th line to promote or demote Global Catalog Server.

Follow the picture also as described below

1.

2.

Every domain controller in a forest stores three full writable directory partitions: a domain directory partition, a schema directory partition, and a configuration directory partition. A Global Catalog is a domain controller that stores these writable directory partitions, as well as a partial, read-only copy of all other domain directory partitions in the forest.

Steps to enable or disable a global catalog

1. Open Active Directory Sites and Services > Sites > site that contains the domain controller that you want to disable or enable > Servers > Domain Controller
2. In the console tree, click the domain controller where you want to enable or disable the global catalog.
3. In the details pane, right-click NTDS Settings, and then click Properties.
4. Select the Global Catalog check box to enable the global catalog, or clear the check box to disable the global catalog.

http://technet.microsoft.com/en-us/library/cc977998.aspx 

http://technet.microsoft.com/en-us/library/cc728188(v=ws.10).aspx