Final answer:
The National Military Command System (NMCS) is the priority command and control structure of the GIG supporting the President, SECDEF, and JCS.
Step-by-step explanation:
The command and control structure designed to support the President, Secretary of Defense (SECDEF), and the Joint Chiefs of Staff (JCS) in the exercise of their responsibilities within the global information grid (GIG) is the National Military Command System (NMCS). The NMCS provides the president and other top officials with the necessary communication and coordination capabilities to conduct military operations. This system encompasses several components including the National Military Command Center (NMCC), the Nuclear Command and Control System, and other facilities necessary for directing operations of U.S. military forces.