[Flatballer]

Gross Tonnage is a confusing thing. It's actually related to internal volume, nothing to do with actual weight. Changing the ballast wouldn't have an affect on tonnage, net or gross. Net is the volume of the cargo carrying spaces (not sure how it's calculated for non-cargo ships), also unrelated to displacement.

Gross and net tonnage are what determines the staffing and regulatory requirements, as well as port fees and stuff for ships.