The London, Edinburgh and Belfast Gazettes, are published by the UK Government and contain a wealth of information useful to genealogists, particularly those researching modern records, as a great number of gazettes from the 20th century are searchable online.
Who puts information in the Gazette? Often it is solicitors, and this is where a wealth of family history trivia can come to light. Personally, I have found a number of name changes, some involving not just the parent but also children, so providing some of the same information that would be in a census, but much more recent. You will often get not only names but an address. Finding a name change helps you ‘kill off’ (or not) someone who seems to have dropped from public view. I have also found insolvencies, bankruptcies and dissolution of partnerships. All useful information.
Everyone has heard of the Bona Vacantia lists published by the Treasury listing the estates of intestate persons, through the ‘heir hunters’ programmes on tv. But even where a will exists, solicitors need to advertise for anyone having a claim on an estate, to ensure that someone having a valid claim is not unfairly excluded, and whilst I believe they do not have to use the gazette, they commonly do (the other place for this is newspapers). A solicitor once told me that publishing an advert in the gazette is like ‘hiding in plain sight’. They do not really expect you and I to read the notices, but other solicitors and heir hunters will. So should all genealogists.
Some awards of medals and honours are listed in the gazettes, usually in supplements issued at the appropriate time. If you had a relative with an honour, that should be listed. You will often get no more than a name and regimental number for medals, unless lucky enough to find an award for valour (VC, for example), and expect to search through many pages to find the subject heading (a weakness of both old and new sites).
The gazette websites are currently being revised, with what they claim is a better user experience in the new (beta) site. Personally I find the beta site looks good, but I have had difficulties downloading PDFs on any but a PC. This will pass, I am sure, but if you search online for “London gazette online”, you will find the websites very easily.