An event gateway is a interface mechanism, in which a ColdFusion application can interact with the server without taking help of the HTTP requests. There are two types of the event gateways -
Initiators -
Initiators are the first event gateways that can generate an event message from an application page and send to user by using an instance. Like - sending SMS notification to the users by using sendGatewayMessage. In it the POP3 server protocol plays a major role in it.

Responders -
Responders are event gateways those respond to a message from an external source by using an instance. The event gateway routes an event to a listener CFC when application developer creates an instance. Depending on the method, CFC can return a response. A listener CFC always responds to an event from a given gateway and processes to it to return response.
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reference links - 

https://alokjasmatiya99.vedamo.com/page/name/lucid+outsourcing/id/28
https://dreamteam.sitelio.me/event-categories