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 3 is an eight-story red brick building adjoining Building 1 on the north. It was completed and occupied in 1940, and though its achitectural lines are in keeping with the oldest building, the lines run more to verticals and horizontals than to the arches and colonnades of an earlier architectural period.