Against the SG rules?

And no potatoes. Never forget about vegetables.

You forgot Desura, itch.io, PSN and XBox Live activate codes.

If you haven't given it away already, here's an idea: make a post about a random but interesting subject but don't say anything about the giveaway in the post. As people reply, you can pick the most interesting/funny comment and check their profile to see if they have the game (or if they met whatever requirements you want). Or you can use random.org and count the comments and pick the lucky winner (and still check the requirements if you want).

Add the code to trade and try and get something you want for it. I don't want it as I have it.

+1

Well that's a shame you can't run it. =/ Maybe hold onto it if you plan on upgrading anytime soon? Or you can do your own raffle in this thread. Have everyone comment, you have a random # picker, ask winner to link their GoG name and see if their library has Witcher 3 or if you can't link libraries, screenshot possibly!

With the forum ninjas, I think it would be pretty hard to sell any game dropped, even GOG keys

Should be able to trade it for other games that you can play here if you don't want to keep it until you can play it yourself. Just be sure to mention it's a GOG code, because there are people that only want their games on steam.

Considering it's something so many people want (I haven't even played 1 or 2 myself however :p), you could hold a small contest on the forum. Have people list why they want it, if they've played the previous ones, etc., then pick a worthy winner and/or use an RNG to select someone from the comments.

trade it on http://www.steamgifts.com/trades.

There's nothing on that domain ! YOU LIED TO ME !!!!

says it doesn't exist

If the new GOG Galaxy thing takes off we may be seeing a site like that pretty soon. :P

Some are being super-helpful because they want a crack at the key. =)
If you didn't have one, you'd be getting lots of comments like, "You should read the guidelines," or "blacklisted for not being willing to read the Guidelines and FAQ for the trading site they're using." :-D