And sure enough, I received a $20 credit to my card and a coupon for a Sirius XM Radio. So far they have been a good creditor, reporting to all of the credit bureaus and I haven't had any trouble. I found a little extra bonus too when I signed up for online statements--I received an additional $10 credit to my card. And also, I have access to coupons for free and discounted food and items in their store.
The only downside I can think of about strictly gas cards is the high APR. Most non-branded cards (without a Visa logo, for example) have a very high interest rate--quite often far over 20%. I don't carry a balance with a gas card, I almost always pay it all off before any interest accumulates.
In addition to strictly gas cards, some Visa, Mastercard, and Discover gas cards have rewards programs such as the Discover Open Road Card. This card gives 5% cash back on fuel and auto purchases (such as repairs and parts). I have found it to be one of the best gas cards out there. However, not all places accept Discover, so that's a bit of a letdown for me. A local place where I frequently stop to get gas doesn't accept Discover.
So, if you're looking for a fairly easy-to-get credit line for building or repairing your credit history (and also some extra neat little perks) turn to your local gas chain. They're always looking for new clients.