A gravity bong is really easy..
Get a Pitcher,1 gallon works good
2liter pop bottle
cut the bottom off the 2liter
Drill a hole in the lid and use a 1/4in drive small socket for the bowl, just make the hole a little smaller, and force the socket in the hole so it fits nice a tight.
fill the pitcher with water and put the 2 liter in it, pack the bowl, lift the 2 liter up as your lighting, youll see it fill with smoke, unscrew the lid and inhale as you use the water for leverage to force it back in your mouth..