Something always happens when you ring the gong, though not everything is immediately noticeable. (For instance, perhaps an elderly villager becomes a child, but looks just like another child you already have, so he/she goes unnoticed at first glance.)

I can't think of a formal list. But here are a few threads discussing different effects:
Gong Grants (spoilers),
(Spoilers) When you ring the gong what happens,
and (spoilers) The Gong Events.

There may be others, but hopefully, these will make a good start for you.