GCWR
The maximum combined weight of the tow vehicle and everything it tows. Towing capacity is this number minus the fully loaded tow vehicle.
In practice
GCWR is the ceiling for everything moving together — tow vehicle plus trailer plus all their contents. It is the number your real towing capacity is derived from, and the derivation is subtraction: capacity equals GCWR minus your fully loaded tow vehicle. Not its curb weight, and not the brochure figure that assumes a base trim with a single driver and an empty bed.
This is why two identical-looking trucks can have very different real capacities. Four adults, a full tank and a toolbox can consume several hundred pounds before anything is hitched, and every pound of that comes straight off what you can legally and safely pull. Weigh the loaded vehicle if you tow near the limit; a public scale costs very little and removes the guesswork entirely.