The reserve fuel tank capacity of Gixxer has not been revealed by Suzuki or any other authority who has been testing and reviewing the bikes. The total fuel tank capacity is 12 liters. You can yourself measure the capacity if the vehicle is available with you by filling 3 liters of fuel in it and driving till the reserve gets activated. Then you can pull the pipe out from the fuel system and empty the fuel in a measuring flask. The empty tank would give you the exact usable reserve fuel in the bike. Sounds odd, but you will get a new figure, which no one else know.

Suzuki Gixxer Series - Gixxer 155, Gixxer SF & Gixxer SF MotoGP Edition