We can extend Multiple Gateway environments feature by utilizing dynamic endpoint capabilities of WSO2 API Manager to have each gateway point to a different back-end endpoint. API Gateway is the actual runtime of the APIs to which are developed and Published from the API Publisher. WSO2 API Manager is capable of publishing APIs to different gateways were the API users connect to those API Gateways in order to do the actual API calls through the applications they are subscribed. Refere Maintaining Separate Production and Sandbox Gateways for more information on publishing to multiple gateways.
However, when the API publisher can only provide a single static Endpoint for an API in the implementation. Therefore which ever the gateway that the API is deployed, the API call is dirceted to a single endpoint as depicted in the diagram below.