Hi Wallace,
Gross tonnage is a measure of cubic capacity, not weight, and I doubt if it is critical for the certificate. If I remember correctly it is calculated out as the cubic capacity in cubic feet divided by 100 and less certain deductions which would probably include the engine room and the fuel and water tanks. If I'm right that would work out at length x breadth x average height x the coefficient of fineness, so say 30 x 11 x 6 x 0.6 (total guesses) = 1188/100 = 11.88 gross tons - engine room and tankage say 2 tons = 9.88 tons.
Gross tonnage should not be confused with the actual weight of the boat, they are entirely different measurements.