Gross tonnage is actually a measure of volume and has nothing to do with displacement. Gross and Net tonnage were used for assessing dues on cargo ships, as they related to the space available for cargo.
If it's in connection with a ship radio licence, I can't recall whether I was asked for tonnage when I sorted mine out; and, if so, how I replied. Gross Tonnage does not appear on the ships radio licence itself.
Dick
Dark Star P275 No 36