Gravity fed hot water is exactly that, when the programmer is in an 'ON' period for Domestic Hot Water (DHW); the boiler lights and a large volume of water is heated up. Because the cold water is more dense than the newly heated water, the cold water falls back down to the boiler, displacing the less dense hot water which is consequently moved up to the cylinder.
Should the room thermostat call for heat (during a hot water period), it simply switches the pump on, circulating heated water through the radiators. As such it is not possible to have heating without hot water.
This type of system can also be used with a Primatic (often fitted with back boiler fires) cylinder or an indirect cylinder. Often no cylinder thermostat is fitted, so the only control for the hot water is the boiler thermostat. This type of system can be expensive to run, however they are extremely reliable.
Some systems do have a motorised valve in the hot water circuit to provide better temperature control. These systems are also slower to reheat the DHW cylinder than a fully pumped system.