I only would like to mention in here (might be some time saver for someone until Sage 10 will be released) that if someone is trying to make custom Gutenberg editor styles work in WP admin, just following this oficial WP codex guide and adding following lines
add_theme_support( 'editor-styles' );
add_editor_style( asset_path('styles/your-gutenberg-editor-styles.css') );
in after_setup_theme action does not work with Sage 9. On the other hand @Simeon solution does fix it indeed. I was bit sceptical at the beginning as this solution here is a bit older and no search results on this topic were pointing to enqueue_block_editor_assets action hook. My fault I did not try it since the beginning, just give it a try it might save you some time researching.
Good luck!