Get first image from Gutenberg-enabled WordPress post

Place in theme functions.php or similar

function get_first_image( $post_content, $size = 'thumbnail' ) {
    preg_match( "/wp:image {.*?\"id\":(\d+),/is", $post_content, $matches );
    if ( !empty( $matches[1] ) ) {
        return wp_get_attachment_image( (int)$matches[1], $size );
    }
    return '';
}

Can be used as a fallback for e.g. if the post featured image is not set, e.g.

$image = ( has_post_thumbnail() ) ? get_the_post_thumbnail( get_the_ID(), 'medium' ) : get_first_image( $post->post_content, 'medium' );
echo $image;