The Global Catalog is a sub-set of the domain itself (and includes any Universal groups in the forest, if it is in one), so a GC can be regenerated from a simple DC. Creating a global catalog is done in the Active Directory Sites and Services tool. This Microsoft article describes it in some detail:
http://support.microsoft.com/kb/313994
Quoting:
To create a new global catalog:
- On the domain controller where you want the new global catalog, start the Active Directory Sites and Services snap-in. To start the snap-in, click Start, point to Programs, point to Administrative Tools, and then click Active Directory Sites and Services.
- In the console tree, double-click Sites, and then double-click sitename.
- Double-click Servers, click your domain controller, right-click NTDS Settings, and then click Properties.
- On the General tab, click to select the Global catalog check box to assign the role of global catalog to this server.
- Restart the domain controller.