In reply to Mahdi.

any theme will and should work with the new editor out of the box anyway

Sadly, some editors add extra components to the editor, which may not be supported under Gutenberg. However, for most themes, that probably is true.

However, there can be more to a theme being "Gutenberg compatible". For example, supporting the wide and full width image/embed option. Unfortunately, many developers are simply using this badge to indicate that it just generally works with the editor.

My hope is that after its release into Core, there will be more standards over what "Gutenberg compatible" actually means.