This project aims at providing a Galaxy-like client for GOG with the help of https://www.gog.com/forum/general/unofficial_gog_api_documentation/page1 also for Linux systems. It will focus the downloading, installing and updating of games at first.
If you need something to play Multiplayer over Galaxy on linux, have a look at the comet project from the brilliant guy who started the unofficial documentation: https://gitlab.com/Yepoleb/comet
There is an alternative for Linux systems written in Python called Minigalaxy:
https://github.com/sharkwouter/minigalaxy
Just download the executable for your os from the GitHub releases. On linux you could use the AppImage, if you want to. It is updateable via AppImageUpdate.
Simply start the downloaded executable.
If you start Andromeda for the first time, you should have a look at the settings of Andromeda. Click on the cog and configure e.g. your desired path for the games here. You can also alter the caching behaviour of Andromeda and define, if you want to update games on startup automatically.
Press the "Install game" button. Type the name of the game or a part of it. You will get a list of all games matching your search. Choose the one you want to install and click "Install".
If everything worked, you should see a status indicator of the download and installation.
To upgrade all of your installed games, press the "Upgrade games" button. If Andromeda finds updates, it will install them. If there are no updates, you will see a notification at the top of the window.
This does not work all the time, but you can try to right click the game and click "Start" to start the game. The console output will be shown in the bottom right terminal. Often execution rights are missing. After you fixed that manually you can use Andromeda to start the game.
For for information about development have a look at README-DEVELOPMENT.