Can I update an existing gate?

Yes. You will need to populate your Inbox with the existing data that has already been collected.

In order to do this you will need an ESRI developer account – sign up for one here: https://developers.arcgis.com/sign-up/

Here is a YouTube video describing how to populate your inbox: https://www.youtube.com/watch?v=aGTPIzTtbSk