Once you have extended GearMessage and made GearMessagePublisher available to the code that will send the message, then you just need to add sending code to the location in the gear where the event occurs.

The following is an example of the sending code, mentioned in Step 4 of Adding Custom Alert Messages to Gears above.

//Create DocumentCreated message
     DocumentCreatedMessage message = new DocumentCreatedMessage(gearEnv);
...
//look up the publisher service
try {
     GearMessagePublisher publisher = getPublisher();
     If (publisher != null)
     {
          publisher.writeMessage(message);
     }
} catch (JMSException jmse) {
...
}