It depends mostly on the construction of your gravity bong. Personally if I were to make one I would use the widest chamber I could find to allow for the water to cool the air around the smoke to make it a bit smoother. A gravity bong obviously doesn't permit the smoke to be cleaned by the water but at least with more cold water under the smoke the air inside the chamber will be cooler and easier to handle. You could also try filling the container of water with ice to get a super cool smoke.