Default Gutenberg CSS on frontend

You can’t. There’s no stylesheet that exists that can be loaded on the front-end to make it match the editor exactly.

Any such stylesheet would need to be created independently of the styles used in the actual editor, because the styles that are used in the editor are written to share scope with the default wp-admin/ styles, which wouldn’t exist on the front-end. This is because the block editor is not inside an iframe, like TinyMCE.

Additionally, any such stylesheet would be useless with all existing themes, because it would not be possible to create a stylesheet that would reliably reproduce the editor styles when loaded alongside theme styles. So such a stylesheet would only be usable as a starting point for new themes. If one were created for that purpose, it would be of little use since themes would need to override those styles anyway, unless they were all going to look the same. So all it would do is complicate development.