Copy gsx.php file into whichever directory you need it in and include it in your code.
To instantiate the object: $gsx = new GSX();
$details = array (
'apiMode' => 'production',
'regionCode' => 'apac',
'userId' => 'user@example.net',
'password' => 'password',
'serviceAccountNo' => '1234567890',
'languageCode' => 'en',
'userTimeZone' => 'AEST' ,
'returnFormat' => 'php' ,
);
$gsx = new GSX ( $details );