We propose a test of gravity on cosmological scales using the newly-defined DES CMASS analogue (DMASS) sample. The CMASS sample is originally designed from the Sloan Digital Sky Survey and provides the most powerful redshift-space galaxy clustering measurements to date. A joint analysis of redshift-space distortions (such as those probed by CMASS) and weak gravitational lensing (currently best measured by the Dark Energy Survey) can provide a powerful cosmological-scale test of General Relativity. Unfortunately, the DES and SDSS-BOSS footprints suffer minimal overlap, primarily on the celestial equator near the SDSS Stripe 82 region. We have built a robust Bayesian model to select DMASS galaxies in the DES footprint specifically to address this lack of overlap. We show that the DMASS sample selected by this model has a fairly good match with the CMASS sample through various validations. To test models of modified gravity, we construct a data vector consisting of Year 1 DES galaxy-galaxy lensing measurements around the galaxies in the DMASS sample and the existing SDSS redshift-space distortion measurements from BOSS.