poudro

One of the content filters is injecting a rogue openning <p> without closeing it anywhere. I don’t know which one though.

If you only have one post with the gallery, a quick fix for this would be :

function remove_extra_p($content)
{
  global $post;

  if ($post->ID == YOUR_POST_ID)
    return preg_replace('/<p>/','',$content);

  return $content;
}
add_filter( 'the_content', 'remove_extra_p', 200 );