Shell's Fuel Rewards Network is an easy way to get free gas. I haven't paid for a fill-up in months. They have a special right now where you need to link a mastercard and you'll get 15 cents off per gallon for every $100 you spend at participating grocery stores (Safeway, Stop & Shop, Ralph's, etc.)

So for every $1000 you spend of gift cards, you'll get $3 off per gallon of gas (limit 20 gallons). This is a value of $60. This is essentially 6% back.

Oh yeah, there's more. The mastercard you link to can be the Citi DoubleCash card, giving you another 2% back.

Oh, and you also get a "welcome bonus" if you go through a referral link. [Referral link removed by moderator pursuant to Flyertalk rules]