If you are on Linux there are three programs you can use:

Lutris
Gamehub
Mini Galaxy

Lutris and Mini Galaxy are primarily for installing but Gamehub does allow to download the offline installers.

There are some community projects available if you search the forums like gogrepo

https://www.gog.com/forum/general/gogrepopy_python_script_for_regularly_backing_up_your_purchased_gog_collection_for_full_offline_e/page1

Any browser will do the job, I use Firefox mostly with no issues, but Chrome is fine as well.
Gogrepo is a python script which will download your library, not used it myself as I have my own setup but it is popular.
You may also be able to use third party plugins to a browser, some don’t work though.
If you really can’t manage the odd download here and there and would rather a third party control your system for you, then there is always galaxy.

Gogrepo, Kalanyr's version. It's via command line tho

eddie3's GOGrepo is considered deprecated, so yes, Kalanyr's version is the best option and it's for Python 3 (Linux and Windows).

Depending what exactly you are expecting the alternative to do, any of these might work for you:

Firefox
Chrome
Chromium
Edge
Opera
Safari
GOG Galaxy
gogrepo.py
gogrepoc.py
lgogdownloader (need to compile it yourself if you are in Windows and not in Linux)

I just download via Google Chrome.

eddie3's version has been updated more recently than Kalanyr's. He is just quieter about updating it.

You have access to all the game files through your library in GOG Galaxy. They would prefer that you use it like Steam, but it can work like the downloader as well.

Thanks, noted.

Yes, I think a lot of people don't understand that Galaxy also downloads the offline installers if that's what you want. It basically replaced the downloader.

Too bad we are being force-fed another piece of so-called "platform" software that offer NOTHING in addition to STEAM, EPIC and all the other useless apps. PLEASE somebody make them agree on a standard so we can choose ONE app to use for ALL games. Im sick of installing as many platform apps as games.

/rant sorry.

Thanks guy that just made your account in the past hour.
Galaxy tries to serve the purpose that you're complaining about, being 1 app that you can organize all your games in on all platforms.

You don't need a client to download your games from GOG.
Just download them from your account here - https://www.gog.com/account

Or, if you don't want to do all manually, as previously stated, you can use gogrepo or even one of its forks, gogrepoc and the version of gogrepo with integrated UI. (sorry, can't go digging any of the links right now). But they work like a charm.

Only problems I noticed are that gogrepo can't download some languages (I think one of the Chinese ones and some other), and it misbehaves when running through WSL on Windows (kinda of a specific case, I know, but the native Python for Windows was misbehaving for me that day "<.< )