I have used google translate and your negative feedback reads" Disk cover damaged during transport because the packaging is too flexible."
Your post does not make clear whether you attempted to compensate the buyer or not. In my experience it is always better to offer a solution as this in most cases avoids negative feedback.
Your post does not make clear whether buyer opened an item not as described case therefore I make an assumption that eBay and GSP do not know of any issues and only thing eBay can see is buyers feedback and according to eBay's this feedback does not violates any eBay policies / does not meet criteria for removal.
I assume you had asked or your buyer had sent you photographs of item they received including packaging. If not then consider asking your buyer if these are available and to send you these and if there is anything you can do to compensate him for the inconvenience caused.
These photographs will provide evidence to you if packaging of your item was changed by GSP.
If the answer is YES then you could appeal the defect and draw their attention to the fact that packaging was changed by GSP therefore damage in transit occurred due to GSP'S packaging being not fit for purpose.
You could also revisit your own packaging of this item and in view of buyers comments determine whether packaging was fit for purpose.
You may or may not have put the CD in a cardboard box and then bubble wrapped it to protect it during transit.
I recently had a case where buyer claimed that an International item had arrived damaged (Case opened). No input from us required or provided and GSP confirmed that when they received the item from us there were no issues and damage occurred whilst goods were in their care, buyers case was closed with a full refund to him and we were not found to be at fault. GSP paid our buyer.
I hope this provides with some opportunity of having negative feedback removed.
