Ofline installers use path you used for GOG versions before. Or rpogram files - where it shoudlnt be installed.

I just posted what worked for me in a thread about Fallout 1. Figured this is the exact same issue, so copy/paste:

1 Give up your attempts with Galaxy.
2 Visit your gog page in a browser, find Fallout.
3 Download the "Offline/Backup" installer.
4 Install Fallout with that. (can even be your regular location for Galaxy games)
5 When done, Open Galaxy, Choose "Gear Icon > Add Games > Scan Folders", aim to the folder where Fallout exists and it'll add it to your list.

From there, you're free to use it as if it was installed by Galaxy client.

Depand on what version you want. Offline installers is only recent ones. GOG Galaxy allow download some else (not every older ones).
Also, as i understand, GOG Galaxy allow to download normal uncensored latest (not downgrade-upgraded) Classic version for owners pf Classic version. While installers - Windows ones - replacet by now outdated Bethesda build something between Interplay old version and current build from non-Classic page (with outdated new launcher+hd-mod).
It should be downlaoded from any 3rd party sites as pre-latest build offline installers or in GOG Galaxy as previous version branch.
And there is problems in finding old windows installers. Saw once 2p2 release, not sure if version is exactly what needed.

Also it can be extracted from Fallaout Classic mac-wine-build. Its not proper native mac version, GOG put windows version there. By Interplay with uncencored content.

In general, GOG use GOG Galaxy as main build. And Offline installer are updated to be compatible with GOG Galaxy backup installation as hush404 say - to be added by scan folders.
And yet. sometimes installer and Galaxy have differ builds included. Even for latest updated game version, if you not looking at older ones. Sometimes even offline have more content that Galaxy build, but usualy it is opposite.

Offline installer IMO better. Evren if you use GOG Galaxy. You always can add them to the client as GOG version or as 3rd party game, same as in Steam.

The Visual C redistributable / it's metadata in the Galaxy Fallout (2/Tactics) repos (at least for the classics versions) is borked and doesn't pass the the test it needs to for GOG to copy it into the Program Files folder.

Somone should probably put in a support ticket so they can fix it.

Its shouldnt be related to Program Files.
Run as admin.

And to be clear, GOG Glalaxy always redownload directx and vcredist (dosbox, scummvm, any else vc\net redists and etc) when you verify game in Galaxy. Galaxy even try to install them again. But if it installed it silencly should be pass after few tries. It shoud be looked for you as latest percentage of verify process, maybe some CMD windows appear-disappear.

(*shouldnt fixed typo)

Ty guys, offline install instantly worked and Galaxy even auto-detected it.
Getting blasted for 190 damage in random encounters as we speak.