I bought a game on GOG recently. It funnelled me hard into getting their terrible launcher and now requires me to start the launcher to play the game, exactly like steam. I'm sure there is some way of avoiding this but it wasn't obvious at all.

Edit: according to this post on the forum the feature is not maintained or supported (as of 2016).

https://www.gog.com/forum/the_witcher_3_wild_hunt/how_can_i_...

The "downloader" mentioned in that forum was a separate piece of software for managing downloads, like a janky proto-steam. To download the offline installers, you just need to go to the games section of your account on their website, and you can click on a game to download the installation files.

Once you have the gog galaxy installed, though, even the "offline" install files of games will automatically get integrated into it. It's horrible.

It is possible that the game you refer to has some form of Galaxy integration (GOG Galaxy has its own APIs) when installed from the client. It might still work if you uninstall Galaxy however.

But Galaxy is optional, you can download the offline installers directly from the site. I have around 680 games on GOG and all of them are in my external HDD in their offline installers - i never installed Galaxy on my PC.

Chiming in to mention https://itch.io

Also Zoom Platform (unrelated to the videoconferencing program, AFAIK they even predate it): https://www.zoom-platform.com/

Its selection is small but it has a few exclusives - Alien Trilogy was released recently-ish there. Also they're the only place where you can find the original Duke Nukem 3D with all of its expansions due to some perpetual contracts they had with 3D Realms before the IP was acquired by Gearbox.