Heroic launcher

Heroic and Lutris are both what you're describing.

I think it may also be possible to install GOG Galaxy in bottles and use that directly, but I've never tried that myself.

I've recently found that Lutris's install scripts haven't been working, at least for the games I tried to install. Not to mention that it seems to take forever to download GOG games.

So I switched to Heroic and I haven't looked back. It somehow downloads games faster than if I got the installer directly from the GOG website, and it has save synchronisation. Some games still require manual tweaks (I've had the most trouble with the classic Resident Evil trilogy), but a lot of them can be done through Heroic's GUI.

When the install scripts are working, Lutris tends to be better than Heroic when it comes to manually installed community patches and the like - you'll usually be given the choice between a script for the vanilla game and one for the patched version (In order to install KeeperFX through Heroic, I had to install it manually and then add it as a library option, for example.)

If you want one that isn't a bloated Electron app, there's Minigalaxy:

https://sharkwouter.github.io/minigalaxy/

Does that exist?

GOG is a store. What you're wanting is some frontend app to do downloads and act as a launcher.

GOG has some frontend, GOG Galaxy, but GOG doesn't do a Linux release of it.

I personally dislike using Steam's client as launcher, and don't like that model (I've got better tools to launch programs and would just as soon not have Steam interjecting itself and would rather not even having it run). One of GOG's selling points is that once you buy a game and download the installer, you don't have any dependency on GOG

if the company goes under, you still can play the games.

For GoG, there's a command-line program, lgogdownloader, that can, among other things, batch-download all your games, but I don't believe that it will auto-install them (and in fact, I don't know if all of the games on GoG have installers that can do headless installation). The open-source, command-line downloader and not having to run software from the company is more-or-less my ideal model, though I can understand wanting to do a headless install.

EDIT: It does look like GOG Galaxy can run under WINE, which is how you'd likely run most of the games you're obtaining from GOG, so you could just use the Windows client in WINE. I suppose that you'd download the Windows client here if you wanted to do that.

EDIT2: The Wine AppDB entry says that you will need corefonts (a collection of Microsoft fonts) to run GOG Galaxy, so if you've never used WINE, setting up a 64-bit WINE environment in ~/.wine and installing corefonts in it will look something like this (on a Debian-family system):

$ sudo apt install wine winetricks
$ winetricks corefonts

Download the GOG Galaxy binary for Windows from its website.

And then run it:

$ wine GOG_Galaxy_2.0.exe