Just purchased Witcher 3 expansion pass, already have Witcher 3 base game installed. Currently getting about 700 KB/s download. Just tested my connection speed at 12 Mbps down.

Installed Galaxy because supposedly that helps. Has no option to install Witcher 3 expansions, only the entire game, of which I have the base game installed.

C'mon, man!

So a bit under half of your top speed (700kB/s = 5.6Mb/s) unless you made a spelling error. Not as good as it could be but not a catastrophy on a weekend.

I suggest you post about your problem downloading only the expansions using Galaxy in the Galaxy sticky thread: https://www.gog.com/forum/general/gog_galaxy_official_discussion_thread