I saw in my game library that 80 Days had been recently updated, but when I went to download the latest version it said that the installer "includes GOG Galaxy client for the full experience and the best support of all features for the game". Now, I've never installed GOG Galaxy because I have no interest in multiplayer, which I understand to be its primary benefit. I also noted that the latest installer is more than twice the size of the previous one, which is not a trivial matter for me since my hard drive space is nearly full.

So what I'd like to know is, why is there no option for downloading the game without the GOG Galaxy client? After all, Galaxy is explicitly described as optional.

More -> Classic Installers. There was a whole optional thread a while back. GOG forbid they actually sent an email or had an actual news post about it.

Found it. Thanks!