The gross area includes exterior wall thickness, and all vertical penetrations (mechanical/electrical, plumbing, elevator shafts, stairwells, etc.), as well as basements and attached garages. This excludes detached parking and loading docks or any structure outside the building line.
See ANSI/BOMA standard* Z65.1 for the detailed calculation method by building type. Gross area is measured to the outside finished surface of permanent exterior walls. The area in green on the right illustration represents the gross area.