Gateway Design Pattern is used to simplify access external resource or API.  You can always simply can work with APIs but such APIs are designed with type of resource into account. Since, native APIs are designed to best fit in their own environment and codes, they may not be much compatible to our platform. Therefore, gateway class is created to encapsulate functions and access to those APIs which is simple object for our development platform.

Refer a picture below which shows how this pattern works..