If you have something in your steam inventory, then you can send it to the other persons steam account. You can keep the GOG key to trade unless you use it.
To check if the GOG key works, go here and put the code in, and redeem it. Then, when the game comes up, it will tell you what it is, and when it expires. From there, you can click cancel to stop the redeem code, or continue (I think) to add it to your GOG library. Please note, if you are wanting to trade a code on that you got from someone else, it may have their username on it.

Want:
STAR WARS™ Battlefront™ II
Dead Space™
Dragon Age™: Origins - Ultimate Edition
Shadowrun Hong Kong - Extended Edition

Have:
https://www.gog.com/forum/general/gogs_general_trading_thread/post23

Feel free to make me an offer. I am willing to trade multiple games.

Indeed. So it's also good practise (and common) to inform the person that you are trading with, that you will not redeem the key yourself but will be using it in a retrade/gift/whatever later.

Want (GOG):
80 Days
Dead State: Reanimated
Indiana Jones and the Last Crusade
Inside
Mainlining

Have (GOG):
Baldur's Gate 2 Complete
Frozen Cortex
Incredible Adventures of Van Helsing: Final Cut
Race Driver: GRID
S2: Silent Storm Gold Edition

You can check out rest of the games I have for trade in my trade list.

Trade list updated

...

HAVE (Steam)
140
Amnesia: The Dark Descent
A Virus Named TOM
Awesomenauts
Bastion
Beat Hazard Ultra
Brütal Legend
Canabalt
Capsized
Cave Story+
Closure
Crayon Physics Deluxe
Dungeon Defenders
Dustforce
Dynamite Jack
English Country Tune
Eufloria
FEZ
FTL: Faster Than Light
Hotline Miami
Intrusion 2
Legend of Grimrock
LIMBO
Little Inferno
Mark of the Ninja
MirrorMoon EP
Nightsky
Offspring Fling
Oil Rush
Outlast
Pixel Piracy
Proteus
Psychonauts
Rocketbirds: Hardboiled Chicken
Shadowgrounds + Shadowgrounds Survivor
Shank 2
Snapshot
Solar 2
Splice
Superbrothers: Sword & Sworcery EP
Super Splatters
Swords & Soldiers
Thomas Was Alone
Tiny and Big in Grandpa's Leftovers
Torchlight II
Trine
Trine 2: Complete Story
UnEpic
Waking Mars

WANT (gog-only; not interested in steam myself)
wishlist

If you don't have anything good to trade me, I'd additionally be happy to just gift you any one of the non-bundle steam codes, if you're a mafia player here, someone who regularly does giveaways (or an irregular one I won something in), or someone who I've positively interacted with frequently enough that you suspect the same might apply to you.

gogtrial34987steam

Thanks for the clarification, and the details from your exchange. Could you, please, post a screenshot of it?

To report them, if you haven't done so yet, contact GOG Support, state your case and provide any supporting evidence you have.

Have GOG's Saints Row 2 and Neverwinter Nights Diamond keys lying around. Will trade for anything cool enough to put a smile on my face. :-)

I think I did the screenshots, let's see

Went to the support center, where do I submit the request? other issues-->other questions?

Hi, just joined.

# have

Rebel Galaxy (edit: oops, misread, it's until 27th june)

# wish

almost anything,
but any sierra / lucasarts / other old click'n point adventure would be nice

please PM with any suggestions!

removed

(irrelevant now)

I have Twitch Prime codes that give some DLC for NBA 2K17, Path of Exile, and Player Unknown's Battlegrounds. Interested in trading for virtually anything.

Do you still have this game for trade? I do have Superhot but not others. I have other games too for Steam

It's not in my library nor is it a key. It's in my steam inventory. Thanks anyway.

That's how I thought it might work. I'm just worried about scammers because they could use the key on their gog accounts if I try to save it for trading. Also concerned people might think I'm a scammer since it takes 24hrs for a steam trade to fully go through, giving either user a chance to flag the trade.

Oh well, I guess I'll just try to be careful. Thanks so far.

HAVE
ARMA: Cold War Assault (STEAM / Windows)
Bastion (STEAM)
Borderlands 2 (STEAM)
Castle Crashers® (STEAM)
Condemned: Criminal Origins (STEAM)
Elder Scrolls III: Morrowind GOTY Edition, The (GOG)
Homeworld Remastered Collection (STEAM)
Nidhogg (STEAM)
Valkyria Chronicles™ (STEAM)

WANT
Dragon's Dogma: Dark Arisen (GOG)
SWAT 4: Gold Edition (GOG)