The Generation IV International Forum (GIF) has selected the Gas-cooled Fast Reactor (GFR) for viability assessment within the collaborative international GEN IV R&D program. The GFR occupies a unique position as one of the GEN IV fast neutron systems, as well as one of the gas cooled systems. The GFR associates therefore the advantages of fast spectrum systems with those of high temperature thermal-spectrum helium cooled systems. The GFR system features a fast-spectrum helium-cooled reactor and a closed fuel cycle. During the exploratory phase of the GIF GFR System Research Plan, the provisional GFR GIF international partners (France, Japan, US, UK, Switzerland and the European Union) agreed to share the work on several combinations of options and to cooperate together at the end of the exploratory phase on a first down selection. The exploratory phase of the design and safety R&D has now produced a down-selection and restructuring of the possible GFR options. This paper summarizes the reference design and the viable "back-up" solutions for each of the major components of the design; the core, the primary system, the power conversion unit and the safety systems.