I am planning a day trip to Gibraltar. Should I consider bring some Pounds, take some pounds from the atm in gibraltar, or rely on euro?
Wikitravel covers this, to some extent.
Essentially, while the official currency is the Gibraltar pound, British pounds are also legal tender.
Most shops in Gibraltar will accept US dollars and Euros
So if you want to be safe, I guess, and if you're going to the UK any time soon, get some British Pounds, as that's legal tender both in the UK and Gibraltar, while the Gibraltar pound is not technically legal tender back in the UK. If, however, you have Euros/USD, they will likely be accepted in most places anyway.
Note, however, if you're considering Euros:
Bear in mind that shops will generally give you a more expensive rate of exchange than the numerous exchange offices and generally won't accept small change. Government offices, post offices and some payphones do not accept foreign currency, and will require you to pay in pounds.
You shouldn't rely on the Euro in Gibraltar because it's not their currency. You will be able to use Euros in some places (not all) but you will get a very bad exchange rate.
It depends on how much you will be spending but either taking British or Gibraltarian pounds with you or taking them out from a cash point will most probably save you money.
It's important to remember that if you take any Gibraltarian pounds home with you that you won't be able to convert them as easily as British pounds into Euros so that may persuade you to take some British pounds with you.
Edit: the use of British in this context referred to the currency used in the UK (or GBP).