There are a two main options when it comes to gas water heating.
Gas Water Heating
Instantaneous Water Heater
Instantaneous gas water heaters work by feeding the cold water through small pipes inside a heat exchanger. The gas flame heats up the heat exchanger, which in turn, heats up the water.

Pro's: Unlimited hot water and saving money by not keeping a large volume of water hot 24/7.

Con's: Limited flow through the heat exchanger, so you will only be able to get so much hot water out of the unit.
Internal/external Gas Storage water heater
Storage water heaters work by storing large volumes of hot water and keeping them hot 24/7.

Pro's: You have access to a large amount of hot water at one time, for multiple outlets.

Con's: Only a limited amount of hot water before it goes cold. Then you must wait for it to heat up again.