you have to read the database table (comprofiler) to get the field value for the current user.

if you have never written a module before you should start studying some of the existing ones to learn.

[code:1]
<?php
defined( '_VALID_MOS' ) or die( 'Direct Access to this location is not allowed.' );

global $my;

$query = "SELECT cb_gamertag FROM #__comprofiler WHERE id = ". $my->id;
$database->setQuery( $query );
$cb_fieldvalue = htmlspecialchars($database->loadResult());

echo 'cb_gamertag value for current user is ' . $cb_fieldvalue;
?>
[/code:1]

the above code displays the value of a cb_gamertag field (assuming it has been created in the CB backend) for the current logged in user and should get you started.