I had decided to make a purchase of Cyberpunk 2077 from GOG.com, but since I have never purchased anything here, I wanted to test the download speeds first. I bought HOMM 2 (size around 500MB) and download speeds were around ~5 MB/s (the B is capital). On steam, my download speeds are generally around ~28MB/s.

So I am just concerned about purchasing CP2077 on Gog now as I am sure it will be a huge game but if the speeds will be this slow, it wouldn't make sense to buy it here. Another point that is holding me back is that the game is about $7 more expensive on GOG.com than on Steam for me.

If GOG decides to fix this problem, I can reconsider buying it from GOG itself.

There is no easy answer to that, because it seems to vary a lot.

I have a 400Mbit/s internet line at home, and what you are seeing, around 5MB/s, sounds about what I get if I'm e.g. downloading a GOG game offline installer with a web browser. So yeah it doesn't seem like I can download "as fast as I can" even though according to e.g. speedtest.net I should be able to do better than that (with an eth cable etc.), and I also recall seeing considerably faster speeds with e.g. a Steam client or the Epic client.

However, just last week I decided to try from my work how fast I can download a GOG game installer with a web browser, and I think I got something like 30 MB/s (megabytes/sec). So much better than what I'm used to seeing at home normally. (EDIT: Then again I don't know if it is possible I was getting the file from some proxy server as some other customer using the same proxy had downloaded the very same file just before; but somehow that doesn't sound quite plausible...)

I don't know why that is, maybe I was just lucky and at work I got connected to a less congested download server, or I just happened to try at a convenient time etc. Either way, there doesn't seem to be any such "hard cap" to GOG download speeds, even if sometimes it feels there is.

Well, I decided to try it now.

Speedtest.net told me I have maybe 300Mbit/s at the moment (my kids are watching Youtube on a tablet and I have some other computers online at the same time as well (like my Raspberry Pi4) and this is over wifi etc.)

Using the Firefox browser on Linux, I downloaded the following file from GOG.com to my computer (from the game Observer):

setup_observer_02.07.2018_(21921)-1.bin (4GB)

This time it seemed to come down as fast I could download, fluctuating between 25-31 MBytes/sec.

So yeah, it depends... I don't know if I'd get different results with the GOG Galaxy client, as I am not using it.

I wish there was some way to download all the game parts on browser at once.

Download speeds on GOG will vary depending on a lot of factors. There's no clear cut answer to address your concern, I'm afraid.

I usually hit my maximum ISP non-local bandwidth when I download offline game installers via browser (around 60MB/s), but there are slow days too.

You can vote for it here:
https://www.gog.com/forum/general/provide_a_full_and_complete_changelogged_download_system

Unfortunately they are only interested in galaxy.

Done. We really need more votes on this.