Having a central Airlock Gateway in front of the Airlock Microgateway is no technical requirement. However, we believe it makes a lot of sense to distinguish between edge-oriented tasks and tasks tightly connected to the services.
The distinction between Airlock Gateway and Airlock Microgateway tasks may vary in specific setups. We recommend using both Airlock Gateway and Airlock Microgateways in a cascaded setup - each with individual tasks and responsibilities.
The Microgateway documentation contains a detailed comparison of Airlock Gateway and Airlock Microgateway.