Well, you have a dilemma. If you choose to wait for the Gateway team to update their product to work on the latest firmware, you run the risk of it never happening, if they can't find a new exploit.
On the other hand, if you choose to sell your updated XL now on craiglist or elsewhere, you still have a good chance of getting a high percentage back of the price you originally paid for it. It's still brand new, you can provide the bill to prove it, and most people are still unaware of the upcoming Gateway card, so don't know they need a 4.5 firmware and nothing above. Wait a few more months and the value of your updated XL will decrease exponentially, as potential buyers are more aware of the "above 4.5" issue. Not to mention the 4.5 units still in stores now will be gone, having sold like cupcakes.
If I were you, I'd bite the bullet and sell at a loss... just make sure the loss isn't too big.