The currency of Gibraltar is the Gibraltar pound (written like the British pound, with the symbol £). Both Gibraltar pounds and British pound sterling are legal tender and are worth the exact same amount when spending in Gibraltar. Euros are not legal tender but are accepted in many businesses. Be warned, the exchange rate used by shops will not be as favorable as the exchange rate you will get if you trade Euros for Gibraltar pounds. Just be sure that you spend any Gibraltar pounds while you are in Gibraltar because it will be difficult to exchange these in any other country except the United Kingdom.
Exchange desks and ATMs are available throughout the peninsula. All major worldwide credit cards such as Visa, American Express, MasterCard, etc. are accepted in Gibraltar, and major UK credit cards are accepted there as well.