After the 1.6.66F5 update on GOG, all DLC is set as uninstalled and cannot be installed.
Thanks for reporting this to us, I've just made a post on the GOG forums regarding this. I'll attach that below, we'll be getting in touch with our counterparts at GOG first thing on Monday.
Sorry to hear about this issue with the DLC on the latest Hotfix. We've seen this issue occur before and we worked with our contacts at GOG to have it resolved as soon as possible.
Essentially for some reason, the DLC occasionally is no longer listed as supported on the new versions, this prompts galaxy to unistall the DLC when it upgrades the base game.
Unfortunately, we won't be able to take any actions until Monday so I'll keep you informed. In the meantime all we can suggest is to rollback to the last working version (1.6.66f3). Once again we're truly sorry for this inconvenience.
We've been informed this should now be resolved :)
Customer support service by UserEcho