My checkout process modified, There're only paypal payment. So I think it cause the script doesn't track purchase.
Here're the code of 'gacode.php' (plugins/system/gacode.php)
$this->purchase = (JRequest::getVar('option') == 'com_virtuemart'
&& (JRequest::getVar('page') == 'checkout.index'
&& JRequest::getVar('checkout_last_step') == 3));
$this->purchase will be tricked if you have correct checkoutstep , i fix code from == 3 to ==4 instead.