There's no separate "Guernsey nationality". There is no Guernsey nationality law. The law which applies is the United Kingdom's British Nationality Act 1981 (formallly, "as extended to the Island(s) by Order-in-Council).

People from any of the Crown Dependencies within the British Islands are full British Citizens. Guersney people may have their passports issued in London, and people from London (if they happen to be in the Island at the time of application) can have their passports issued in St Peter Port. The only distinction which exists (which is of little practical consequence) is that a minority of people from the Crown Dependencies have restricted EU free movement rights, under the provisions of Protocol 3 to the treaty of Rome.