Grargar: But this doesn't make any sense. There are already games with serials here, like the Neverwinter Nights series.
undeadcow: I'm not sure... but isn't Neverwinter Nights serial necessary for multiplayer only? Maybe GOG takes a stand against DRM serial activations for single player component but is tolerant of multiplayer keys?
Not really. I have Neverwinter Nights 1+2 on disc and both of them require serial keys for their installation, in the same way that Titan Quest does. GOG automatically installs a fake serial key for Neverwinter Nights 1+2, but it only works for the single player part. If you want to go multiplayer, you have to ask support for a key for the first game, or use the key that is attached to your account for the second game.