So the villiagers have just constructed the gong with all 4 peices. Now what? How do I ring the gong? What does the gong do? What is the purpose of the gong?

To ring the gong, you place a villager on the gong. The gong does a lot of things. Oh, it's too hard to explain! Just search for 'Gong' in the search of the forums and you'll find all the info you need.

Is the Gong "good" or "bad" you decide

I don't know who put "[SPOILER]" infront of my title, but this is not a spoiler. It doesn't tell me anything.

You may look here:
http://www.ldwforums.com/ubbthreads/ubbt...=true#Post78797
and there is many other treads!

The gong can gave you good things like food,village,etc.But it can gave you bad things like lost all food,3/2 villager,etc.