The API provider is responsible for creating and deploying an API on API Gateway. Each API is defined by a file written as an OpenAPI 2.0 spec.

The OpenAPI spec defines the publicly facing URL of the REST endpoint for the API, the backend service accessed by the API, and any other characteristics of the API such as authentication, data format, and response options.

The API client makes a REST request to an API hosted on API Gateway to access backend services. An API client can be any app capable of making a REST call, such as a browser, mobile app, or web app.

The API client only needs to know the URL of the API, the request verb (such as GET, PUT, POST, DELETE), any authentication requirements, and the format of any data sent to or received from the API.

https://cloud.google.com/api-gateway/docs/architecture-overview#:~:text=API%20Gateway%20is%20an%20API,Service%20Management%3A%20for%20managing%20API