So you WERE able to use the GC to upgrade tickets to park hoppers? I ordered base tickets with my package, and had asked the CM if I could upgrade to PH tix when I check in, using the GC, and she said that I couldn't, b/c it couldn't be used to pay for any portion of a package. But since my package will be paid by the time I check in (since it has to be paid 45 days in advance), I WON'T be using it to "pay" for my package, but to pay for extra add-ons to tix.
You didn't have any trouble doing this? Did you do it at resort check-in?