Broken implies shattered in pieces and not chipped or marked.
If item was broken before it got to GSP or they reported the break to you as they received it then this is between you and your postal service provider and does not involve GSP.
If item has reached your customer overseas and they are reporting it as broken then you need to obtain photographs and then ask the buyer to contact eBay's CS and advise them that shipment was through GSP and that buyer has a problem and they will guide him on how to proceed.
I would expect your buyer will be told to open a case reason: Broken or similar. Advise him not to use any other reason like not as described.
You could advise your buyer to do this without him speaking to eBay and once case is opened you will receive a message from eBay advising you that your funds are on hold and you will be contacted if eBay requires any further information.
Depending on GSP records your buyers claim will be accepted or denied by GSP and none of this should concern you and once case is closed I would expect your amount on hold to be released back to you.
Hope this helps.
