Chapter Zero is the demo of Goodboy Galaxy, the events in this story immediately precede the full game. The demo is freely playable on GBA (via flash card) or emulator. There is a web-based player on the demo's itch page.

https://goodboygalaxy.itch.io/goodboy-galaxy-demo

All of the characters in the demo return in the full game.

The events of the demo are summarised in comic form in Goodboy Galaxy's manual: Read the comic