GPM value or gallons per minute value is the gallons of water a water connection point uses. To calculate the GPM value you need, you have to check the number of water connections you want your water heater to provide hot water. Now separate those which are in daily and continuous use. Then calculate the GPM value of each connection. If you are not aware of the GPM value of different water connections in your house like the showerhead, dishwasher, sink, etc. then these statistics might be helpful for you.
Standard Hand Sink – 0.5 GPM
Washing Machine – 1 to 1.5 GPM
Kitchen Sink – 1 to 2 GPM
Dishwasher – 1 to 2 GPM
Water-Saver Shower Head – 1.5 GPM
Standard Shower Head – 2.0 GPM
BathTub – = 4 GPM
Whole-Home – up to 6 GPM
Calculate for yourself and match it with the product you are going to choose.