The instantaneous gas geyser is plumbed in-line into the existing water piping network. Effectively you connect a cold water supply to the inlet, and a hot water draw to the outlet. If you are replacing your existing electrical geyser, you take the cold water going into your existing geyser and reroute it to the new gas geyser, and do the same for the hot water supply.
It is recommend that you put the new gas geyser as close as possible to the outlet where the most hot water is likely to be consumed. This reduces the pipe length and the time you wait for hot water to arrive at the water outlet i.e. tap and also reduces the loss of heat due to exposure to cold air through the piping in the winter months.
The installation of the gas component bears a little more consideration due to the flammable nature of the product. Fortunately there are clear best practices which are supported by a regulatory set of laws that dictate the requirements for the safe operation and installation of a gas supply to a domestic dwelling.