There are several cards that earn 5% back on gas. Quite a few people on FT carry the older PenFed Cash Rewards card for 5% on gas, paid each statement.

However, unless you buy a lot of gas, you'd be better off looking for the best signup bonus across all cards. For example, there are several cards offering bonuses easily worth $750+ right now. Will you be buying more than $15k of gas in the near future? That's something on the order of 125 tanks for most people.