Hello Ganesh,
As per my understanding, a 3rd party developer may develop their own APIs on their endpoints, and publish the APIs through Developer portal and just route to their api endpoint(s). And the gateway owner usually develop/test APIs on the gateway, and then publish to Developer portal (set it as managed by Developer portal).
The gateway-portal integration supports both ways. And I don't see the difference between the APIs published by these 2 ways.
Regards,
Mark