The definition of gas is a little bit more clear cut though there is some confusion among some cardholders.
Under the Amex definition, you will earn the extra cash back at "standalone gas stations". That means that most national recognized brands count. For example, Shell, BP, Sunocco, Lukoil, Gulf are all considered as standalone gas stations as their main business is well, being a gas station. They may sell snacks in their "store", but their main line of business is gasoline sales.
However, there is another group of gas stations that are not considered as "standalone". This includes stations that belong to discount stores, superstores or warehouses. For example, Costco has their own gas stations at certain stores (they do not have diesel). Their prices are often slightly lower than other regular stations. But if you use this card at Costco's station, all you will earn is the regular 1% cash back. That is because Costco station are not considered standalone. Instead, their main business is being a discount warehouse and they are classified as such under the MCC (merchant category code). However, if you have the affinity card from Costco, then you could earn 3% cash back at their stations as well as other standalone stations.
The same applies to Walmart gas stations. Walmart also has their own stations at some of their stores. Using this card would not get you extra cash back. Instead the regular 1% applies. For you to save money at Walmart's stations, you need one of their credit cards or their Walmart Money Card.
The same thing applies to other non-standalone stations like Kroger etc.
So as long as you are filling up your gas at regular stations, this card should work just fine in terms of getting you extra cash back and savings. You only have to be aware that some stations like Arco do not take American Express! But they do take discover!.