National Harbor is accessible via Metrobus from the Southern Avenue Metro Station (Green Line) and King Street Metro Station (Yellow/Blue Lines). Buses run every half an hour and drop off at two stops, both about a block from the Gaylord. WMATA also offers a trip planner if you want to get information on your entire trip.
There is valet and self-parking available at the Gaylord. In addition, there are several other garages around the National Harbor area. Information is available on National Harbor website's parking page.
National Harbor is accessible via the Potomac Riverboat Company water taxi from Alexandria, VA.