You don’t necessarily need the API Gateway. When you have a REST API Descriptor, the path to interact with your API will be rad/[path to the REST API descriptor] by default. So, for example, if you consider the following package:

The URL path to interact with the greetings API would be rad/pc.greetings.API:greetings, meaning a consumer of this API would be able to interact with it by using the base URL https://server:port/rad/pc.greetings.API:greetings.
Now, that is a somewhat ugly path to expose to external clients. Plus, it gives away too much detail about the underlying implementation. So, in the absence of an API Gateway, I like to use the Integration Server’s URL Aliases feature to abstract the actual path from the consumer. For example:
Now, consumers can use the friendlier URL https://server:port/API/greetings instead.
Planning these friendly URLs ahead of time is useful as you can include them in your API definition (e.g. Swagger) from the get-go. Now, in order to use URL aliases in this manner, the IS extended setting watt.server.url.alias.partialMatching needs to be set to true.
Hope this helps,
Percio
PS. The TECHcommunity editor makes every occurrence of the word ‘API’ uppercase automatically for some annoying reason but imagine the paths above have ‘API’ as lowercase to match the screenshots. 
#webMethods#API-Gateway#Service-Designer#Integration-Server-and-ESB#API-Management