GG and JE are open domains
The information from Peter E is wildly incorrect. That is to say, WRONG!
The domains are /entirely/ open and are s not regulated, in the way that housing occupancy is in some of the Islands may be. Even housing is not regulated in the Island I live in (in reality things are nothing like what you might believe from watching reruns of 'Bergerac'!!!). The registry is, like .UK, run by a private company not the States.
Anyone can register domains and there are various domainhack uses, including quite a popular application in Holland and Belgium for JE (anyone who speaks Dutch will understand why).
If you register directly at
www.channelisles.net is, as other posters have said, (relatively) expensive but not dramatically so than Nominet's direct fees (49 GBP per year, no VAT).
The natural audience is not exactly "tiny" but it is admittedly "small" -- a total of 120,000 souls for the Islands, compared with 60 million for the UK.
And by using registrars (popular EPP connected registrars are gandi, bbonline, microtech, blacknight), registrations are much cheaper.
I hope this information corrects some fundamental misunderstandings about the Channel Islands' TLDs.
It is first-hand information (I work for the registry) and you can always email or contact us (we are a small team of 5) via the contact details at
http://www.channelisles.net/contact.shtml
(Incidentally, in case anyone is interested, SEDO will be auctioning GG.GG this month.)