A domain name is an address on the internet. The two uses you'd be most familiar with are web addresses, where a domain name is prefixed by "www" and email addresses, where the domain name follows the "@" symbol.
What is .GB?
.GB was a reserved country code top-level domain (ccTLD) for the United Kingdom (namely Great Britain and Northern Ireland). It was never widely used.
Can I register a .GB domain name?
Unfortunately not. It is no longer possible to register under this domain.