There isn't one. The GOG version is DRM-free, so there is no key and you don't need one anyway.

As LaithArkham said, there's no key. Just download the game from GOG (from the "My Account" link), run the installer and you're good to go! You can keep that installer file and install again as many times as you want without ever needing to come back to GOG, but if you ever do need to download it again you can do that as many times as you like.

Also check the extras, which include the soundtrack and level editor as well as wallpapers and other artwork.