Clicking play just plays the game 
Are all games closed source here?
The engine and all examples included in the main example list within the engine are open source.
Gd.games is a game hosting platform, not development platform, and the games within it are not modifiable.
It is up to each developer if they want to make their game open source or not, and then they would need to host the project files for download. Some do this over on itch.io, but by default it is up to each dev.
oh ok