It is curious to me why GCHQ didn't just contact GitHub to acquire github.com/gchq but instead decided to go with the long and cumbersome github.com/GovernmentCommunicationsHeadquarters. Perhaps it is a British thing [1].
GitHub explicitly has a Name Squatting Policy [1] that states:
> Account names may not be inactively held for future use. GitHub account name squatting is prohibited. Inactive accounts may be renamed or removed by GitHub staff at their discretion.
I have been able to acquire a couple of GitHub names that have been inactive for several years by contacting GitHub support, and they usually reply within a day or so.
[1] https://www.addedbytes.com/blog/if-php-were-british/