It sounds like you're trying to pay for the order with 2 cards almost; I'm not sure if our payment system can handle that. This may have to be done manually since the GC doesn't cover the cost of your order. You can either call/email CSR and get them to do it for you, or place the whole balance on your CC, then in the comments section state how you wish to apply the $100 GC, provide all the necessary info to charge it, and they will recredit the $100 to you CC after charging the GC.