Fix Wordpress gallery showing only to logged in users

functions.php

function quickhack_fix_gallery( $query ) {
  if ( $query->get( 'post_type', '' ) == 'attachment' ) {
    $query->set( 'post_status', 'inherit' );
  }
}

add_action( 'pre_get_posts', 'quickhack_fix_gallery', 99 );