I save 19% on mine by buying them at Kroger. I do it when my Chase Freedom has 5% cashback at grocery stores (April-June this year) and when Kroger has a 4X fuel points special. On a $1000 purchase, I will get $50 cashback from Chase plus $140 in free gas. I put the $140 I save on gas toward paying for the GC.
I'm glad to see this thread. I usually charge to the room and pay off the night before checkout. But when I take my whole family or when my DH and I start going for longer trips (thus spending more money), I'll make sure to pay periodically throughout our stay to avoid a CC charge.