A gross ton was a non-mass related nonlinear unit of measure, used to determine a vessel's overall internal volume.
Montgomery Scott expressed his frustration following the rescue of Harry Mudd's ship Stella, at the cost of several of the USS Enterprise's lithium crystals, of how dire it was that "almost a million gross tons of vessel depending on a hunk of crystal the size of my fist." (TOS: "Mudd's Women")