Mr_Whiffles: If a game had exclusive pre-order DLC (or a goodie that was removed), isn't it technically possible to acquire these items from someone whose account contains them because GOG is DRM free? I was wondering this because a new competitor to Steam touts the ability to resell digital games, which if extended to all digital content concerning the game would be a tremendous improvement for people such as myself who would like to own complete games, but GOG is DRM-free, so that creates problems, leading to the alternative described above. What do you think?

JMich: No. Bonus codes do expire though.
A gift code is a code bought as a gift through the checkout screen. A bonus code is a code provided for free (possibly along with other stuff). If you paid GOG for the code, the code shouldn't be expiring.
Another difference between bonus codes and gift codes is that gift codes can be partially redeemed, while bonus codes can only be redeemed once, and any games not redeemed from them are lost.
It used to be easier to distinguish between bonus and gift codes due to their format, but that no longer seems to be the case.

Mr_Whiffles: Well, If GOG no longer has the authority to sell a game, why can I still add a de-listed game to my account, years after the fact, through gifts?

kohlrak: I'm betting you have a specific case in mind. Giving us details on this specific case might allow us to develop a precedent. Out with the details, because we can't really debate a topic if we can't even agree to the terms used in the topic.

Maighstir: As long as copyrights are being held, and the copyright holders have not explicitly made it freely available, making an unauthorised copy is piracy ...