GSA stands for General Services Administration, a U.S. government agency responsible for managing federal buildings, supplies, and transportation services. It also provides technology and procurement services for other federal agencies.
It's likely that the GSA logo was custom-designed using Frutiger Bold, a simple and highly legible sans-serif typeface created by Adrian Frutiger in 1976 for the Charles de Gaulle International Airport's signage system. You can learn more about the font here.