There is some key details here...

https://forums.inxile-entertainment.com/viewtopic.php?f=43&t=20484

"For those using GOG Galaxy, we have reports of success when unchecking "Update to the latest version automatically", which then allows you to manually update to the latest version."

If you need any further help please feel free to contact our support team directly at https://inxile.zendesk.com/hc/en-us

We don't all use Galaxy. There's no patch available for offline installers.

Exactly. Being able to download offline installers (and patches) is the primary reason I purchase games from GOG. If I have to rely on an online service to install and update games then I'll just stick with Steam where games are often cheaper.