Have you tried refreshing your account? You can do it with this link
https://www.gog.com/user/refresh

Also is this for the offline installers or for gog galaxy?

If you still can't figure it out you should contact support
Or if support takes too long to respond since they are busy you can send a PM to one of GOG staff
https://www.gog.com/u/ponczo_

Thanks for the reply. I did try refreshing my account multiple times, but no go. This is for the offline installer. thanks

Hopefully you got it figured out. I just had an issue where the windows download was actually downloading the mac installer, however refreshing the account fixed it.

GoG eventually fixed it. Took over a week though.

I'm glad it was fixed at least even if not all that quickly :/.

I got the mac installer for the DLC again when I downloaded the update for the expansion yesterday (just the previous backgrounds DLC not the expansion, if the developers are reading please bring the same launch discount as Steam to GOG and I'll get that one too :) ), but switching to mac and then back to windows was enough to be able to download the correct DLC file.