Never dealt with GSP. Hated paying the extra when sellers really didn't have to use the program. Unless GSP for some reason requires that you ship it back, I wouldn't do it. The seller also submitted photos that they packed the item properly, meaning the seller isn't claiming accountability for it. You should be looking to get in touch with eBay since they promised you the refund. Follow up with them. Call if you have to and get a case number each time you communicate with an agent. Get their name and date of when you spoken with them. If you do a live chat, get a copy of the transcript if possible (or copy & paste the text conversation. Screenshots are also good too). The last thing you want to put up with is the "we don't have record of this, we never said that" line. If they're professionals, this shouldn't be too bad, but cover yourself so that you have something to show and hold them accountable for what they said.
Keep us posted.
Almost a month on, and I finally got this sorted.
I contacted eBay, and they responded saying that they have given me the refund, and asked me to contact PayPal if the money's still nowhere to be found. They gave me PayPal's customer service email address (which is somehow not shown anywhere I can find on PayPal's website), which I used to contact PayPal. After 3 rounds of exchange in emails, I was asked to start an instant conversation online so they can verify that it's me who wanted to check on the status of the refunds. After several more rounds of exchange via PayPal's own instant messaging function, I finally got the message that the refunds were processed and sent to my card. But wait, I didn't see any payments made to my card, last time I checked? I checked my card's statement again, and finally figured out what happened. Turns out that PayPal has sent the refunds to my card a month ago, but my bank's credit card account aren't showing any of those records until the latest statement recording them comes out, hence why I'm only seeing those refunds on my card's statement just now, and the false impression that the refunds weren't credited to my card previously.
With that said, PayPal's (and eBay's) services aren't exactly impeccable. At the beginning of my email exchange with PayPal, I was told that I was not using the registered primary email address linked with my PayPal account to contact them, and so they cannot respond to my queries. But that exact same address was on the row denoting who the sender of the original copy was at the bottom of their response. Their website's design was also a bit non-intuitive, not only for the absent customer service email address as mentioned above, but also for the fact that the cross on the instant messaging tab actually ends the conversation entirely instead of just closing the tab. It also surprises me that eBay won't let me leave a feedback on the seller some time after I've asked for a return & refund through the Resolution Center, because the seller actually did a pretty good job in handling my queries and was quite cooperative throughout the Resolution Center process, and I'd have left a positive feedback if given the chance. But it was over 90 days since the transaction, so I'm probably at fault for not giving feedback earlier.
Okay, after all this crap-talking, I should probably give my opinion on GSP. Would I buy something that is only shipped via the program? If it's something ordinary and cheap, maybe, because I think you're gonna receive the item as long as you filled in the correct address, but I'm not sure about that. If it's a limited edition of something expensive, then probably not, because the process, from applying for a refund and return to confirming that you have received the refund, is quite tedious to me. If it's something that's also fragile, then there's even less of a reason to do so, because there's a high chance that the courier will break it during shipping. Item is broken, and you wanna return it for a refund? That's just gonna increase the chances of the item being further damaged. Although I probably shouldn't complain, considering that I don't have to return the item, and now have a free model car.