Hello,
It's probably a GOG code - you can send an email to
support@indiegala.com with details of the trade, the time it was completed, and what games, and they can give you the person's IG profile where you can see their forum name ( and Steam account if he has it linked ) to contact the person you completed the trade with, and since that's the user's responsibility and he should offer you a replacement or the key if he is a honest trader.
I advise you to read every trade's description and title before sending an offer, double-check the user's profile, and if the trade or the thing he may ask is realistic, since there have been users in the past who scammed people.
Here is a possible example for this trade
Mount & Blade II: Bannerlord - Trade 293231 | IndieGala Trades, and here is his profile
Yabun | Indiegala, which is most likely a scammer since he asks for an expensive game, very cheap games, and has no rep. Sometimes, even users with rep scam people since they fake the rep by having multiple accounts, so the best advice is to be careful, and even sometimes to ask other trades/forum users if they had a bad experience or trades with a certain user.
Welcome to Indiegala trading!