Travellers will undergo two passport checks, with Spanish frontier officials having the final say on entry based on Schengen requirements for passport validity and length of stay.
British passports must be valid for at least three months beyond the planned departure date and have been issued within the last 10 years.
Future requirements will include facial biometrics and fingerprints once the EU's entry-exit system is implemented.
Time spent in Gibraltar will now count towards the 90-day Schengen limit within any 180-day period, ending the previous allowance for extended stays.