WordPress: Add body class (for single post template) or post class (for blog/archive pages) "made-with-gutenberg" for posts/pages created with Gutenberg editor.

add-gutenberg-class.php

// add class to body class on single post template
function themeprefix_body_class_blocks( $classes ) {
    if ( is_singular() && has_blocks() ) {
        $classes[] = 'made-with-gutenberg';
    }
    return $classes;
}
add_filter( 'body_class', 'themeprefix_body_class_blocks' );


// Add class to post classes in blog page or archive pages.
function themeprefix_post_class_blocks( $classes ) {
	if ( is_archive() || is_home() ) {
		if ( has_blocks() ) {
			$classes[] = 'made-with-gutenberg';
		}
	}
    return $classes;
}
add_filter( 'post_class', 'themeprefix_post_class_blocks' );

Based on the code example in WordPress/gutenberg#4418 (comment)

Try on your own risk :)

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