If you are looking for an instant gas card, you should either consider gas station credit cards from a particular brand or prepaid gas credit cards.
Gas station credit cards can be usually purchased inside a gas station or online and function only at that particular brand gas stations. They are easy to be approved for and usually, offer instant decision.
Featured: Credit card for complicated credit situations
Prepaid gas cards are very simple and serve as a substitute for cash. You just buy them in a grocery store or a gas station and use at the pump.