Edit: according to this post on the forum the feature is not maintained or supported (as of 2016).
https://www.gog.com/forum/the_witcher_3_wild_hunt/how_can_i_...
But Galaxy is optional, you can download the offline installers directly from the site. I have around 680 games on GOG and all of them are in my external HDD in their offline installers - i never installed Galaxy on my PC.
Its selection is small but it has a few exclusives - Alien Trilogy was released recently-ish there. Also they're the only place where you can find the original Duke Nukem 3D with all of its expansions due to some perpetual contracts they had with 3D Realms before the IP was acquired by Gearbox.
youtube-dl is amazing. It supports a lot of platforms, and you can download entire playlists by just using the URL to the playlist itself.
Against the ToS? Perhaps. But it's my book, damnit.
Only buy, fix and support those who have your ideals.
This is the case with virtually everything these days. Stallman is/was right about this. You only rent the movies on Netflix, the games on Steam, the books on Amazon Kindle, and so forth. Everything could be taken away from you for absurd reasons. Who would have thought that your account could be disabled for reasons that it could get disabled today? It is not going to get better. You cannot even play a single player game offline, without running Steam. Torrents/Warez will never go away thankfully. Screw DRM anyways.