Find source of annoying gong sound
This is not a answer, but I have to use it to explain the process.

It is difficult to find out using the description of a sound like "gong".

You can do some investigating by playing system sounds then reporting which one was it.

To do that go to following folders:

/System/Library/Sounds/
/Library/Sounds/
/Users/&lt; username &gt;/Library/Sounds/


Now just click on the Space bar to have the QuickView play it.

Start with the System/Library.

I would guess the "gong" is the Sosumi sound. But you can enable the screen flash that will flash the screen when a system sounds occurs..

Open Syst pref and click on Accessibility. Now click on the Audio and select the Flash the Screen.

Also go to your Notification center and turn of sound for apps one by one to find out which one is it, if it comes from there.