That's quite an undertaking.
So the link you provided has a bit of a walkthrough in its description about where to start. The trick is that because you're trying to program a notification service for an EXISTING app, you don't really have control over the notifications the app itself creates. I suspect what you're going to have to do is program a NotificationListenerService, listen for gmail notifications, and somehow cancel the gmail notification and replace it with one of your own, as created through the android documentation.
For a good example of how NotificationListenerService works please take a look at this:
https://github.com/kpbird/NotificationListenerService-Example
The canceling is something I have not tested but you asked for ideas, not code. NotificationListenerService has a method cancelNotification(String pkg, String tag, int id)
which looks like you can use to cancel the gmail notification.