I assure you, you are not going to get help from Gunbroker. I ASSURE YOU. I've been there. You will quickly learn that in all the mumbo jumbo, gunbroker just pretends it is a clearing house of items, like craiglist. It in no way is an Ebay that will stand with you against a false description. No way, no how.
Gunbroker is the WORST when it comes to a problem.
However, you credit card WILL take care of you. Everything in the world is marked up to cover credit card fees. Part of those fees cover benefits provided by credit cards. Dispute resolution is one of those benefits you pay for every time you shop at the grocery store.
If you have even a mid credit card, they will give you back the disputed funds the second you file a dispute.
This was my experience with GB as well. Don't bother chasing them down a rabbit hole if they don't help you, which they wont. There are lesser known but better auction sites out there. People who get burned by GB and get sour on all online transactions don't realize that though GB is the biggest auction site, it's also the worst in terms of buyer protection and customer assistance with disputes, in my opinion.
I've had this same scenario using another auction site and though they couldn't force a refund, they could and did put a hold on the seller's auction account until the dispute was resolved. They are also much more proactive about policing for scam auctions and responding to and investigating complaints.
I'm sorry to hear about the op's troubles. My recommendation is to wait for GB's answer on the off chance they might help with a refund, assume they wont, and start a CC dispute immediately if that's how you paid.
If you didn't use a CC and the seller won't voluntarily refund, congrats. You have a new 629.
Count yourself lucky, you actually got a gun for your money. I didnt, seller told me I'd have to wait at least a month for him to restock the gun listed as "In stock" before he could send it, and to top it off was removing factory accessories in his auctions from guns and selling them seperately. When I refused to wait a month for an incomplete gun, I had to pursue a refund for months, with no help from GB and a seller who degenerated into a lunatic sendi g me emails mocking me, saying thanks for the free $1100. Finally after getting the BBB and his local LE involved he refunded me with "a $200 restocking fee or no refund at all"...for an item he never had in stock.
Gunbroker ONLY response to all this was to delete the negative feedback I left for the seller.
The seller remains on GB to this day, with great feedback and a terrible BBB review.
Next I'd start looking into either attempting to reseat the sideplate yourself, or getting a new side plate from somewhere like numrich.