Please ask your buyer to send you photographs of what he received and a clear photograph of label and customs declaration form that was attached to his item , then discuss this with eBay's CS resolution team.
GSP did ( Or Should) check the item they received against order and have details on record as they had to complete a customs declaration on your package, In my experience chances of GSP getting packages/ Items mixed up is unlikely but not impossible.
It is just as likely that mix up occurred in his country's customs , buyer is making a false claim or getting his orders mixed up.
Hope is that your buyer is claiming that what he received is completely different item (ie Clothing instead of Shoes) and GSP records show HS Code which will confirm that item they received was correct to order.
I would not ask buyer to open a case of any kind till I have spoken to CS.
If buyer opens a SNAD case then system may hold you responsible and that would increase your difficulties in having this corrected/ case decided in your favour. When a case is opened eBay will put your funds on hold and this is standard procedure and nothing for you to worry about.
Act on eBay's advice and you should be fine.
Good luck.
