George C. Marshall National Historic Site Study Act - Directs the Secretary of the Interior to conduct a special resource study of the Dodona Manor and gardens in Leesburg, Virginia, the home of George C. Marshall during the most important period of his career. Requires the Secretary to: (1) evaluate the national significance of the study area and the surrounding area; (2) determine the suitability and feasibility of designating the study area as an affiliated area of the National Park System; and (3) consider other alternatives for the preservation, protection, and interpretation of the study area.