tfishell: DRM-free. (You should know that by now! ;) The only key might be a multiplayer key if the game had multiplayer.
archisbhatt: Well usually games that originally require CD keys do come with purchases but you don't need them to play the game. GOG has given CD Keys for a number of games I own. As far as I know this game needs a CD key for multiplayer, unless EA retired it like it did for MoH, Clive Barker's Undying and a few others (Hence they have no keys on GOG).
When a cd-key is necessary for single-player games gog uses one key for all installers, because it's easier for us (no need to enter anything), easier for them (no need to organize different installers and cd-keys) and for the publisher (no need to generate new keys). If a game needs a different key for multiplayer gog can give out additional keys by request (there should be an info on the gamepage, as example Painkiller) or they list them in the download area (as example Heroes of Might and Magic 5).
About SimCity 4: On the gamepage there is the info that SimCity 4 is singleplayer only, so it's very unlikely gog got more then one key from ea. I already own the game drm-free from gamersgate and there is also no key necessary to install and play the game. You can try and ask the support for an additional key, but I'm pretty sure they can't help you.
