[teddyh]

No. The official TLD – i.e. “Top Level Domain”, is .uk. It differs from the normal ISO 3166 two-character country code, GB, even though most other countries have an identical TLD and ISO two-character code. The TLD .gb is merely reserved to deter shenanigans.

[Symbiote]

Both .GB and .UK are the country code top-level domains; .GB because that's the official code, and .UK for legacy reasons.

UK is a reserved ISO 3166 code, to prevent complications with DNS TLD assignment.

The .GB TLD is in use, but there are only three known domains. This is one:

  host hermes.dra.hmg.gb
  hermes.dra.hmg.gb has address 146.80.9.16
  hermes.dra.hmg.gb mail is handled by 10 relay.dstl.gov.uk.

[justincormack]

There were some mil.gb domains at one point.