The process Payment (used by on-site payment) method executes before an order is placed while the PostProcess (used by redirection payment methods) method executes after an order is placed so you can get the order by the GUID only in the PostProcess method.
PostProcessPayment is invoked once the order information is saved in the database after you hit the confirm order button at the last step of the checkout.
ProcessPayment is invoked before the order is saved in the datbase