So, I forgot to turn off automatic updates, and Gog just updated me from 1.2 to 1.3 and thus broke all my mods and wrecked my story.

I know I can have an "offline" version as well, but I didn't download (or save) the 1.2 installer.

Anyone have any suggestions on how I might get the 1.2 installer again?

Cheers!

GOG doesn't provide any method I know of, unless support can help you.

It's a bit late now, but this is why it's useful to download offline versions from the account page, especially for games like RimWorld. You can make sure you have access to a prior version if anything breaks.

"Automatic updates" suggests you are using Galaxy. Galaxy can install older versions. At least it used to have that feature. I think it was under the "settings" for the game last time I was using it but I can't remember the exact name. Look around on the "page" for Rimworld or consult some help document about Galaxy to find it.