I am going on a holiday to Spain and Portugal and I wish to go to Gibraltar for 1 day. I have been granted a multi-entry Schengen visa. Do I need a visa for Gibraltar too?
It depends of your nationality
*Nationals of China (People’s Rep.), India, Mongolia and Russia holding valid multiple entry Schengen visas can enter Gibraltar without a visa for a max. stay of 21 days, or “where the multiple entry Schengen Visa is due to expire before 21 days from the entry of the person into Gibraltar, the remainder of the period of validity of such multiple entry Schengen Visa less 7 days.”
For others must be issued Gibraltar visa. More information
Your Schengen visa is probably not valid. Gibraltar is a British Overseas Territory and thus not part of the Schengen area.
There is a list of countries which need a visa, so depending on where you are from, you'll need to get an extra visa. Note that if you have a multiple-entry visa for the UK that was issued for at least six months, you don't need to get a new visa.