gravity hot water system

Does anyone know what the gravity hot water heating system is all about?someone called me to get rid of the old boiler . I already installed a hot heating system with a pump- size of supply pipe 31/2"large- return pipe 3" size. can someone explain how the system works- there is a new b&g100 pump could this pump work with the new system?  could I reduce the size of the pipe from 31/2" to 31/4". the customer only wants to change the boiler and not all the pipes in the house