Global Catalog only store user & computer objects info and their permission/rights. in a domain, you can have different computer hardware to store same Global Catalog., so if one domain controller is down, you can login the domain with other domain controllers that are still running, still able to access resources that's probably stored on other servers.
That's why MS usually suggests that you have multiple domain controllers and seperate them from application & storage servers.
Cluster in Microsoft term means you have to setup two computers with same hardware spec but share the same hard disk. So if one of PC of the cluster is down, you still be able to use the data or application on the server.