Ebay automatically put your funds on hold if a buyer opens a case.
As long as you can prove delivery to the hub, as you assume, you are not responsible if the item then gets lost.
Use the link below to contact Ebay Customer services.
Explain that you used GSP and it got safely to the hub, and ask them to close the case and release your funds.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)