No visa is required for UK citizens or EU citizens that are in possession of a valid national identity card. Here is a list of the countries whose citizens require a visa. Travel visas are valid for stays up to 3 months.
Should you have any travel concerns, you can resolve them in the UK at the Passport Office in London’s Globe House in Ecclestone Square. Once in Gibraltar, certain enquiries can be answered by the Immigration Department in New Mole House, which can be reached via e-mail at firstname.lastname@example.org.