The difference between GA-8413 and GA-8403 is the gas connection.
GA-8413 is for the German market and works with 50mbar and has a ¼" gas connection.
GA-8403 is for the rest of Europe (except France) and works with 30mBar and has a 8mm connection.
In terms of power, it is 2*1.5kW and 1*1kW=4kW in total.