The one time GSP repacked a shipment to me, they included a letter (on eBay letterhead) saying they did it.
And with damage in transit, the carrier (Canada Post, CanPar, UPS etc.) usually will also include a letter as well as new packaging-- like a clear bage over the whole mess.
GSP is not known to accept damaged shipments at their doorstep in Erlanger KY, so that damage happened post -GSP and they should be taking responsibility.
Be sure to state that the shipment was via GSP. Yes, it's on the paperwork, but help the processor to speed things along.
Tha being said, did the seller ship in the store display package wrapped in a black trash bag? Neither of those are designed to withstand ordinary treatment in transit. The trash bag is obvious, but the display carton is meant to be pretty and to ship on a pallet with dozens of similar boxes.