I've installed the Gutenberg Editor module and have it working with various content types, however I'd like to also use it to create content for custom blocks (as in structure > block layout > add custom block). I've tried googling but since Gutenberg uses the term 'blocks' to describe the sub-modular pieces of content it uses, I can't find any reference to the module itself being available as an editor for custom blocks, or another module that extends Gutenberg's functionality to custom blocks (or any block element).
Currently, it's not possible. Although Gutenberg module provides a Drupal text editor plugin, like CKEditor, it relies a lot on the node edit form to provide the editing experience.
I am the project owner and lead maintainer for the Gutenberg module.