On the AA website it says:
"Note: Gift Cards are redeemable toward the purchase of air travel wholly on flights operated by American Airlines, American EagleŽ and oneworldŽ carriers for itineraries sold and originating in the U.S.,"
That sounds pretty limiting for someone based in Singapore unless they're book a one-way return from the US perhaps?
Meanwhile the Delta gift card FAQ says:
ARE GIFT CARDS AVAILABLE FOR PURCHASE INTERNATIONALLY? A US billing address is required for all gift card purchases on delta.com.