GPO is the federal government’s primary centralized resource for gathering, cataloging, producing, providing, authenticating, and preserving published information in all its forms.
GPO is responsible for the production and distribution of information products and services for all three branches of the federal government.
Additionally, from The WPA Guide to Washington D.C. (1942):
Building 4, on the east side of North Capitol Street and facing Buildings 1 and 3, completed and occupied in 1938, is commonly designated as the GPO Warehouse...
The warehouse (Building 4)...is a long rectangular three-story gray cast-stone building facing the main GPO group, and extending back to the railroad tracks behind Union Station. The building is 467 feet long and 87½ feet wide, its floors designed to carry a load of 500 pounds to the square foot. A spur of trackage enters the third floor of the Warehouse, and there is space for 16 freight cars on this floor. Seven hundred carloads of paper, each weighing 40,000 pounds, can be stored in the warehouse at one time. Thousand-pound rolls of paper are unloaded from the freight cars and carried by an automatic "lowerator" to the floor below. Electric trucks, tractors, trailers, heavy-duty cranes, elevators, and other rolling stock are necessary in the movement of paper between the warehouse, through a two-way underground tunnel 30 feet beneath North Capitol Street, and the several floors of the main plant.