Damage in transit through the GSP has nothing to do with you, the seller. Just keep out of it, eBay's timelines are their concern. If your buyer wants to complain it's up to them.
Incidentally, your buyer is likely to be kept waiting much longer than if you were responsible for refunding them. EBay is almost instant when requiring sellers to refund buyers, but they seem to take conspicuously longer when it's eBay itself that may have to meet the cost!
Concentrate on making sure that the claim is dealt with by eBay and not dumped back on you as the seller.
