When you buy a game you sometimes get store credits, these can't be traded. Sometimes you get a free $5.99 code from GOG when buying games like Witcher 3, these codes are tradeable and so are any gifts you buy from the store.
Thank you for explaining that. I was confused because I remember seeing the word codes in context of GOG credits and later on all I saw was that difference being added to store credit. It's all clear now.
To others, I'd be interested in a $5.99 code and am open to trade offers for anything you want on steam.