Enebias: It depends. If you bought it, it will still work. If it is part of the bundle released for free the last winter, it will not. The codes have been disabled by the GOG staff because a few users abused the system: they took hundreds of codes with the purpose of reselling them later.

EDIT: oh dear, I think that with those flash giveaways I will have plenty of work to do! ;)
By the way: does anyone know if I can convert my game (Omerta) in a gift code? I didn't have it, but I'm not so interested in it either...