Regarding the confusion with the codes, there are 2 types of codes. The gift codes, with content that was paid-for and the bonus codes, with content freely given by the staff.
Bonus codes have a limited validity, the expiration date info comes with the code. Gift codes are valid until redeemed, that is, they do not have an expiration date.
EDIT:
Quoting JuriJ's response to my enquiry:
Hello
Bonus codes have time limitation as they usually connect with some promo action, so we want them to be redeemed in a proper time. Gift codes as they are paid by users and supposed to be a gifts do not have any time validations.
Regards,
JuriJ
GOG.com Support
Post edited December 31, 2012 by Densetsu