You always have to ask yourself if you have entered into anything that could have won you a gas card, if not then its needs to be ignored I would think.
bobbie; we don't have 'gas' (meaning petrol) or dollars in the UK. That's a big giveaway, plus the fact that you haven't entered any competition to win the money. My advice is to mistrust any e-mail that seeks to persuade you that something good and unexpected has happened to you out of the blue; unless you actually know the person who is mailing you.