ELTs equipped with MCAO systems will be powerful astrometric tools in the next two decades. With sparse-field precisions exceeding 30 uas for V > 18, the ELTs will surpass even GAIA's per-epoch precision for faint stars (V > 12). We present results from an ongoing astrometry program with Gemini GeMS and discuss synergies with WFIRST and GAIA. First, we present a fit to the relative orbit of the individual L/T components of Luhman16 AB, the nearest brown dwarf binary known. Exploiting GeMS' wide field of view to image reference stars, we are able to track the relative motion to better than 0.2 mas. We find that a mutual Keplerian orbit with no perturbing planets fits the binary separation to within the measurement errors, ruling out companions down to 14 earth masses for certain orbits and periods.