Payment Gateway redirect/iframe to/from payment provider

I’m developing a payment gateway extension and it “works” in the sense of I can show it in the backend and frontend and have my “Place Order” button.

But that’s where I despair..

I want to:

1. Via API Call, create an order at the external payment gateway backend (I can do that with curl in the Gateway Client
2. When I receive the order URL from the external payment gateway, I want to redirect to this URL so that the customer can pay there. Alternatively, display the URL in an iframe so that the customer can pay.

How can I do step 2? I can’t find that out.. Both options would be ok for me..

Thanks a lot in advance!