It's your internet connection. I'm way further from Poland and I get better speeds.

That seems quite fast imo.

they probably don't have a server in your city or in a nearby town

Yep, depends on what time of day, what zone (country), servers and issues related to Sales ... BIG Sales being the worst, and then for a while afterward as many try to download their games ... many having given up during the sale.

It has been worse for many people since COVID, so no doubt bandwidth with the number of users also plays a part.

The ever increasing size of many games doesn't help either.

I finally found out what is the issue on my PC: I have MSI Dragon Center installed, which by default installs a program called "cfosspeed" that is accessible via "LAN Manager" in Dragon Center.

Once I uninstalled cfosspeed, my download speed on GoG is back to 100%

Interesting. Thank you for sharing. I still have it installed, but I've always kept the Windows service disabled. Maybe that's why I don't have any issues.

Ironically, it's supposed to make the connection faster:
cFosSpeed increases your throughput and reduces your Ping. cFosSpeed uses Traffic Shaping to reorder Internet data packets in such a way that urgent traffic is transferred first and the rest of the data later. This results in significant reduction of latency (lag) and typically also results in connection speed up.
-- official site

Maybe something to consider enabling if I ever get into multiplayer games where response time is very important. It's staying off for the foreseeable future though.

Maybe it considers Steam traffic more urgent than GOG traffic?