The Roman Galactic Exoplanet Survey (RGES) will detect >1000 planets with masses as low as Mars mass around G,K,M stars. The high resolution capability and high precision photometry of the telescope will also yield the mass measurements of 700+ such planets and their host stars during its prime mission. This talk will focus on precise photometry and astrometry of planetary hosts and background source stars at very small angular separations. This talk will also show demonstrations with precursor programs using Hubble, Keck and Gaia. These RGES mass measurements for microlens host stars and planets will aid in estimates of eta_earth by interpolation between RGES and Kepler results and understand the formation of cold planets orbiting host stars with varying masses.