Change City label to Suburb

When you copy a function from the documentation you don't need to copy the opening <?php and ?> closing php tags as they are usually already present at the top and bottom of functions files, so your file should look more like this

<?php
/**
* @package WordPress
* @subpackage Default_Theme
*/

if ( function_exists('register_sidebar') ) {
register_sidebar(array(
'before_widget' => '<li id="%1$s" class="widget %2$s">',
'after_widget' => '',
'before_title' => '<h2 class="widgettitle">',
'after_title' => '</h2>',
));
}

add_filter("gform_address_types", "aus_address", 10, 2);
function aus_address($address_types, $form_id){
$address_types["australia"] = array(
"label" => "Australia",
"country" => "Australia",
"zip_label" => "Postcode",
"state_label" => "State",
"states" => array("NT"=>"NT", "ACT"=>"ACT","NSW"=>"NSW", "QLD"=>"QLD", "VIC"=>"VIC", "WA"=>"WA","SA"=>"SA", "TAS" => "TAS")
);
return $address_types;
}

add_filter("gform_address_city", "change_address_city", 10, 2);
function change_address_city($label, $form_id){
    return "Suburb";
}

?>

Regards,
Richard