When it comes to the name of the inventor of gravity bong, nobody really knows. The science behind gravity bong, however, can be traced back centuries ago.
Asia is attributed with wide spread and common use of water in smoking devices during the Ming Dynasty of the 16th Century, but some protest that it was in fact Africa where the first type of water smoking pipe was used.
Sourced from African Smoking and Pipes:
Previous studies of African smoking devices wrongly assumed that all smoking pipes radiated from the West Coast of Africa beginning when tobacco arrived on the continent. It was assumed that tobacco smoking spread across the savannah as a substitute stimulant for kola. Thus, whenever archaeologists found pipes in Africa, they automatically thought that the pipes were from 1600 CE or later. If this were true, it would mean that any African water pipe would be too young to have been invented before the hookah. But it was false. This theory completely missed the African cultures in the east and south that had been developing smoking devices before the arrival of tobacco. See complete article here.
Gravity bongs came out in the late 90's as a party trick for stoners. When someone has a bong or pipe, the intention may be to only use it for smoking legal tobacco. But if even on a single occasion, that bong or pipe is used to smoke cannabis, then the pipe or bong now is illegal.
The drug paraphernalia statute, U.S. Code Title 21 Section 863, makes it “unlawful for any person to sell or offer for sale drug paraphernalia; to use the mails or any other facility of interstate commerce to transport drug paraphernalia; or to import or export drug paraphernalia.”