ChesHatter: I basically just downloaded the Installer again, but since I don't see any duplicate files, can I just assume the setup file has been updated successfully? Do I need to run setup again?
As was mentioned before: only the Mac version got updated in this case so if you're a Windows user nothing should have happened this time.
Anyway, yeah, in case of GOG there's so far two ways how games get patched. Normally an "updated" flag should appear next to the corresponding files so you know which one it is.
a) The entire installer gets updated. The version number changes and downloading the game again results in a new installer file (or several new installer files - if it's a larger game that additionally to the executable comes in BIN files) with the new version number. In these cases you *should* uninstall and reinstall the whole game to be safe.
b) A patch is added to the downloadable files. Just download that one and execute it and the existing installation will be updated accordingly.
If you add a game to the downloader that has already been downloaded in the current version the downloader should only do a quick scan and only redownload it if there's either been at least small changes (which for some reason did not result in a higher version number) or your previously downloaded files are corrupted.
Anyway, if there's been an actual notable update you should always end up with a new file with a higher version number.