The Merchant Marine of a nation is a force of ships operating under civilian banners in peace time and, in times of war, commissioned by the naval branch of service to augment the main surface fighting force. As such, Merchant Marine strength plays a key role in determining the overall naval firepower of a given country. In war time, commissioned Merchant Marine vessels would be charged with transporting soldiers, vehicles, supplies and the like to points anywhere in the world.
For the purposes of the GFP ranking, landlocked nations are penalized for the lack of a Merchant Marine force as landlocked nations have shown to be capable of operating a Merchant Marine force through ports in allied countries (as exemplified by Austria).
Sources: CIA.gov, CIA World Factbook, wikipedia.com, public domain print and media sources and user contributions. Some values may be estimated when official sources are lacking.