You needed an NIE number
As a British Citizen you can move to Gibraltar whenever you want (whenever you can afford to - I compare property rental and purchase costs to that of Brighton in the UK) and stay as long as you want straight away on your British passport.
However, if you want to register there as a 'citizen' then you need to produce 3 month's worth of Utility bills and proof of income if you are still working. If retired, you need to sign your health and pension over to the GHA (Gibraltar Health Authority - their version of the NHS) but if you are still working you need to arrange private health insurance at the cost of approx £4k PA. Assuming you are successful then you will receive a blue ID card which identifies you as a resident of Gibraltar but NOT as a full-on Gibraltarian. This card entitles you to certain benefits including free bus travel and entry to libraries, museums etc, which non-residents and tourists have to pay for. It is THIS card that I will be entitled to if (when) I retire and move out there. However, at present (since Brexit) the blue card does NOT currently entitle you to move freely in the Schengen zone, whereas the RED card does. I'm hoping that this will change with the deal currently being thrashed out between Gib, the UK and the EU. Spain and Gibraltar are keen to allow them, the EU is currently not and the UK doesn't give a flying FK because stopping free movement was one of Brexit's main aims

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The RED card identifies you as a full citizen of Gibraltar and you NORMALLY get this after 10 years residence - I say 'normally' because there are ways to get them quicker - it's a small place and it depends on who you know and/or how much money you have - if you have £2m for example, you can get one straight away. Sadly that is beyond my financial capability.
There are other coloured ID cards too but I'm not sure who gets those - military personnel perhaps? Contract workers there for a short period? Spanish workers who work in Gib? I don't know - and it's difficult to find an exact description of them on the Internet - I've tried!!!!
(Sorry for distracting the thread away from LJ........)