Looking at the git, the imports have been excluded, which means you will need to open the project once with the Godot Editor before being able to launch it directly (developers can automate this in a build script, see --no-window
).
I haven't looked much into it, but the license used is the GPL3, and the README mentions links to Steam's proprietary API. This is likely not the intent of the author, but it seems you would need to remove the Steam integration to comply with the GPL (if you wanted to fork the game, not reuse single files, and the author is unaffected, of course).