On Steam there's something called an "Escape from Castle content update," but I don't see anything about it here. I don't know if this is related to the new map editor and Steam Workshop integration, but if it isn't, then GOG users should receive the update as well.

They updated it here too.

Great! Where can I download it? I looked for an update or a patch and I can't find it listed as a direct download or through the GOG downloader.

Well I use Galaxy for the updates so dont know if you need to download the whole game again. I see on browser that the setup comes in 3 parts so maybe part 3 is the update?

If you use Linux, there's a patch available for download. Otherwise, if you use the Windows or Mac version you need to redownload the whole game. No, part 3 is not a standalone installer / updater, it is a part of the main installer and all parts of this needs to be downloaded if the game is to be installed/updated on Windows.

I tried the osx version... but there are no new levels