Then you must have some form of identity proof i.e. an ID card or a passport
Then you need proof that you are living in Gibraltar, i.e. a rental agreement or property deeds.
From the Government website:-
Change of Possession of Motor Vehicles.
The change of possession of motor a vehicle requires that both the seller and the buyer complete their respective form. This is a requirement that is set out in law.
At the time of submission the applicant must provide proof of identity and of residential address to the satisfaction of the Licensing Authority in support of the application.
The following documents shall be admissible to ascertain proof of identity of an applicant:
a) Passport; b) Identity Card; c) Gibraltar Driving Licence
There is a guy called Ivan Zillah Williams, I think, who has a 2nd hand car dealership just outside Casemates Sq, in Coral Rd I believe, try googling him...But Gib is expensive, better to buy and drive one down....