Question: What’s the Gutenberg development process?
Answer: It’s complicated.
Gutenberg code is managed using Git. The Javascript code is built using node.js. Some of the PHP code also gets built. Developers create branches in which they develop fixes to issues or new solutions to satisfy the requirements. Changes are submitted as Pull Requests which are reviewed, run through automated code quality and Continuous Integration tests and reviewed by peers. New releases are created fortnightly.
Unless you’re a developer or someone who’s been asked to test a fix you probably won’t need to use the Gutenberg source.
See Gutenberg plugin
References
Block Editor Handlbook: Tutorials