Because Galaxy doesn`t download compressed installers or patches, but all files. It usually replaces files that are different to the ones in its filelist. So it should happen that the 10GB are way less and the download seems "very fast" because it only downloads some files. Remember its still beta and patching comes in next release and compression maybe in a later one.

Sounds very plausible but is unfortunately seems to be incorrect. It is downloading the whole thing or at least 300MB at a tiny percent before I decided to abort it.

GOG Galaxy is still beta software. By using it we're helping with development.

There is some workaround (it worked for me when I've played The Witcher 3):
http://www.gog.com/forum/the_witcher_3_wild_hunt/the_gog_galaxy_nightmare_with_patch_110/post4

Download 300 MB patch and install it and then try Galaxy. If it will try to download entire game then use above method.

Other way is patch manually and use this one:

http://www.gog.com/forum/general/gog_galaxy_beta_2/post2930

I don`t use GOG galaxy. I left the likes of Steam and Origin, UBI etc, exactly because I don`t need these platforms and their hassle. Been happy with downloading and installing manually.

At least GOG gives us the option not to use it, unlike the others.

I still like the old GOG installer the best. I tried Galaxy once and decided I didn't like it. It may be that over time it will be improved to where I will prefer it. In the mean time, I hope they keep producing GOG installer files. So far, so good.