Currently in file action.gateway_complete.php (Orders version 1.6.12) method
ProcessGatewayResult is called after getting the order object that is sent with
event "PostGatewayComplete". The method ProcessGatewayResult makes final changes
to the order so the beforehand acquired order object for event is not valid.
e.g. the order status in event is still proposed instead of paid.

So the suggestion is to send with that event valid order data.

Secon suggestion is to put up this event only when payment is completed and
order is paid.