GPM stands for Gallons Per Minute. Therefore, it's simply a tankless water heater than can provide 5 gallons of hot water per minute.
Tankless water heaters heat only as much water as needed, when it is needed. They heat water on demand and use 24 to 34% less energy than traditional water heaters. They are made for applications whose consumption of hot water is more than that of residential use.
You can count on these units to provide as much as 15.2 liters of hot water per minute. Most of them come with a warranty of more than 20 years. Their shelf-life can be further extended due to these heaters having replaceable parts, which you can easily exchange when they go bad.