GRT meaning in Military and Defence ?

Gross register tonnage (GRT, grt, g.r.t., gt) or gross registered tonnage, is a ship's total internal volume expressed in "register tons", each of which is equal to 100 cubic feet (2.83 m3). Replaced by Gross Tonnage (GT), gross register tonnage uses the total permanently enclosed capacity of the vessel as its basis for volume. Typically this is used for dockage fees, canal transit fees, and similar purposes where it is appropriate to charge based on the size of the entire vessel.

Net register tonnage subtracts the volume of spaces not available for carrying cargo, such as engine rooms, fuel tanks and crew quarters, from gross register tonnage.

Gross register tonnage is not a measure of the ship's weight or displacement and should not be confused with terms such as deadweight tonnage or displacement.