Using Gutenberg without WordPress

It’s Open Source code, so you are free to fork it 🙂

The new block editor was specifically built for WordPress though and as such it is integrated with many other things, including the Media Library.
Getting it to work without the core code would likely an undertaking of mammoth proportions.

Drupal even integrated Gutenberg as well – https://www.drupal.org/project/gutenberg So there are definitely people doing this kind of non-WordPress integration with the code base Gutenberg provides. Good luck!