PAPER their offices with GWs. I mean, decide on a time-frame that suits you (like, say, "I'm going to devote 4 months to getting this stupid thing GWed" ) and then send them one every other day for that length of time. Sooner or later, they'll get tired of your harassing them and give in.
Buy oodles of colored stationery and mayhap spritz them lightly with nice perfume. Anything that will make YOUR GW stand out.
Another idea is if the card is closed (I think now, that I might have read that), wait a teensy bit and open a new account with them. They'll try harder to make a current customer happy (and possibly give a GW) than they will an EX-customer.