I don't have Costco AmEx. I find it has no value at all comparing with the Fidelity AmEx card, which is 2% everything without any annual fee. For gas, you should use a 5% cashback card such as PenFed. So I would suggest you to cancel the card.
At the end of your 12 months membership, if you have a costco AMEX card, they will extend and charge your card for 1 year automatically. So how do you manage to get the extra month essentially? And then how to pay with the certificate bought here rather than letting them charge your AMEX card?