Omnipay gateways that redirect offsite don't necessarily need to call "complete" methods.

[bummzack]

A gateway that redirects offsite doesn't have to implement authorize and purchase in pairs of authorize + completeAuthorize and purchase + completePurchase. It's possible that a Gateway might omit the complete… methods, if there's no data coming back from the offsite request.

Implementation of AuthorizeService and PurchaseService needs to check supportsCompleteAuthorize and supportsCompletePurchase respectively during initiate and complete the action immediately, if the complete… method isn't implmented.

Reference: thephpleague/omnipay-paypal#170 (comment)