Willy, the purpose of elevating the heater is to preclude the heater pilot light or burner from becoming a sourse of ignition to gasoline fumes that may occurr in the garage.
Just an FYI, the Military owns one hell of a lot of water heaters that run on gasoline.
Further FYI, thanks to the BRILIANT AlGore who repealed the Laws of Physics and made all of us so much safer, any new gas water heater you buy will soon come equipped with a hydrocarbon sensing gas valve. This will assure that you continue to pay welfare to support the 50 aproximately stupid people who burn themselves and their houses up each year by storing gas cans near the water heater. Don't you feel much better now.