Trading on GOG (though I've only traded once, over a year ago):
You cannot trade any games that are in your account.
If you have not downloaded anything from a game in your account, you can send a request to support to ask them to convert your game to a gift, which would then be tradeable. They may or may not be able to help. If done quickly after the purchase, I think you won't have any problems.
To trade a game, you must have a gift code. When you purchase a game you can choose to purchase it as a gift. You are asked to submit an email address for the person the gift is for - use your email address that is associated with your gog account.
In your account you can select unredeemed gifts to see what you have available for gifting or trading. The redeem codes are found there with your order.
The person with lowest rep sends the game code first via chat message.
So let's see if I got this straight:
- find game you want
- purchase game
+ make sure you check the gift code box at checkout
+ use your own email address to gift to
- now you have the game and a gift code to trade with?
Also, I cannot trade a game for which I do not have a gift code.
Sounds ok, but is this ok with GOG?