Why not just roll them over and use a GC to buy another one when the opportunity comes up?
How are you cashing out the gift cards? (I got a $100 gift card from eBay a couple of months ago for selling something, which i bought with a different account. I then got the offer on the new account, which i used the first gift card to pay for. The same for the third... It's effectively buying a $100 gc for $13.20, and then getting $86.80 cash for each successive transaction.)