Show Block only if it's a front page and GET variable is set
You missed a closing bracket. Try this:

&lt;?php
if (drupal_is_front_page() &amp;&amp; !empty($_GET['listed'])) {
  return TRUE;
}
?&gt;


or

&lt;?php
if (drupal_is_front_page() &amp;&amp; isset($_GET['listed'])) {
  return TRUE;
}
?&gt;