Honestly the way GOG galaxy 2.0 handles downloads is a huge PITA .I was downloading offline installer of a game from galaxy and after downloading about 2gbs data ut of 15 gbs the Download speed got stuck at 0.1mb/s to 0 mb/s , i restarted the galaxy client and boom the download was gone from the downloads and the funny thing is in gog galaxy directory if i open the game temp folder there is a huge 15 gb game files which obviously don't work consuming space for no reason at all . Honestly i love GOG and there client but never in my life i had issues on steam , Uplay or origins regarding the download speeds or handling the game installation files.

Uninstall galaxy, then download offline installers from the web page. Works great, and you get to choose whichever browser you want (or command line) without being tied into some proprietary tool. I download some 80gb+ just the other day (serious Sam 4 plus a load of updates) through Firefox, took a while but no issues.
Client free, wins every time!

For some reason downloading through browser is quite slow for me, not sure why though since other gaming clients give good download speeds at around 10 MBps while I get like 0.3MBps through browser. Also, maybe I'm too much of a boomer but I'm constantly afraid of downloads failing midway through browser download. Tried going back to Downthemall for Firefox but its nowhere near as good as it used to be. I'm kinda with OP on this one. Curious about how others are handling these issues, in case they do have them.

Change your browser then, that’s another benefit of not being tied to some proprietary client. If your getting less speed through browser, then it sounds like the browser is broken, galaxy doesn’t magic you up any special extra bandwidth (although I would be suprised if offline downloads get throttled or something to show how we should all use galaxy).
As for downloads failing, haven’t seen that in years, more likely internet drops out completely. If gog provided decent options on downloads:
https://www.gog.com/forum/general/provide_a_full_and_complete_changelogged_download_system
The. You could torrent. However the only focus is on galaxy.

Checked with another browser, speeds are still bad. I have no idea what to do now, shooting an email to support to see if anything materializes that way. :(

Edit: Galaxy meanwhile still downloads at 11 MBps when I click install game but speed is super low (~ 0.2 - 0.4 MBps) when I click download installer through Galaxy. Seems very strange to say the least.

Yes, don’t know then. I noticed that over the last few days there have been several posts on download speed from various countries, so perhaps there is some issue there. Hopefully gog can support you on it.

Well, GOG doesn't want you to download the offline installers. They (still) keep them available, but they make it as uncomfortable as possible to actually use them. So of course installing through Galaxy will be the preferred way while the use of offline installers is discouraged by removing the GOG Downloader and stalling the download speed in Galaxy.

Howere, as of now, there are still ways to get the offline installers in a convenient way that GOG hasn't closed yet:
1. You can use a third party download helper. For example the Free Download Manager: https://www.freedownloadmanager.org/
2. You can use gogrepo: https://www.gog.com/forum/general/gogrepopy_python_script_for_regularly_backing_up_your_purchased_gog_collection_for_full_offline_e
2a. If command-lines aren't your thing, you can use the gogrepo GUI: https://www.gog.com/forum/general/gogplus_download_checker

Ah, I am something of a Python man myself... will check that out. Thanks!

I was pretty disappointed to discover GOG Downloader had been sunsetted. It is what it is though. Hadn't been around much for past couple of years and was just using my existing installers so I am still catching up on changes here.


Edit: Even gogrepo cli tool gives similar speeds, I think maybe its a technical issue with their CDN or something than them throttling speeds intentionally. Let me see if support team gets back with any answers.