Gross tonnage and displacement are often confused, and they are decidedly not the same thing.
The displacement of a vessel is defined as the weight equal to the weight of the water it displaces when it is put in water. That displacement is the weight of the boat. This basic principle goes back to the time of Archimedes.
Gross tonnage refers to the interior volume of a ship in units of 100 cubic feet. But if we go back to medieval times, when wine was the most important cargo of the day, all shipping fees and other freight charges and duties were based on the ship's capacity with respect to how many casks of wine, called tuns, it carried. A "100 tun" ship could carry 100 barrels or tuns of wine. It became the standard of measurement. Tun became ton.
Each wine cask (aka barrel aka tun) held 252 gallons of wine, which happens to weigh 2,240 pounds, or one imperial "long" ton.
Centuries later, the world needed better measurement tools, so admeasurement was substituted for wine barrels, and capacity calculations replaced units of tuns with units of 100 cubic feet. This was formalized by the newly formed organization that would later become the IMO.
Today, a ship's gross tonnage is the volume of its interior in units of 100 cubic feet.
Have a great week.