Thank you all for reporting the issue. Let me quickly explain what happened, and what are the next steps for this.
The codes you're all talking about are the ones you received during giveaways, and you already own those titles in your GOG library. The codes were sent out via an automatic email, which truly didn't have an expiration date there, but it didn't mean the codes will never expire. To add another layer, the redeem page actually stated that the code will never expire, which is a bug on our side. Sorry for that confusion.
We're now working on fixing those issues so with next giveaways you'll know what is the expiration date of every code you receive.
As for the latest codes you got, so the ones for Fantasy General, Mount & Blade and Constructor Classic, within the next days we will extend the expiration date, so all of you can redeem those codes. I'll let you all know in this thread when the codes will be valid again, and for how long.
Hope this solves the issue. And once again sorry for the confusion.
Explain the reasoning behind the expiration with knowledge that it was a giveaway and it has to be assumed that the current and fresh coming userbase will all have the game and the use of that code would actually only be a long time after said giveaway when enough new (community active!) ppl came which have not participated and could/would be willing to trade it with another gog game.