GECCO is a unique concept of combined dual mode γ-ray Compton telescope with coded aperture mask. It will conduct high-sensitivity measurements of the cosmic γ-radiation in the energy range from 100 keV to ~ 10 MeV and create intensity maps with high spectral and spatial resolution, focusing on sensitive separation of diffuse and point-source components. GECCO utilizes a novel CdZnTe Imaging Calorimeter as a standalone Compton telescope and as a focal-plane detector. It provides detection of incident photons with a position resolution of < 1mm and with energy resolution of ~ 1%. A coded aperture mask modulates the incident photon flux and creates an image on the CZT detector plane. The mask is deployable and can be placed at a large distance from the detector for high-angular resolution. A plastic scintillator anticoincidence detector (ACD) above the CZT Imaging Calorimeter provides protection against charged cosmic rays. Side BGO octagon shields and bottom BGO shield provides absorption of natural background photons, and vetoes productions of background photons by charged cosmic rays. CsI log calorimeter located under the CZT Imaging Calorimeter measures energy escaping from CZT. GECCO combines the advantages of two techniques - the high-angular resolution with coded mask imaging, and a Compton telescope mode providing high sensitivity measurements of diffuse radiation. With this combined “Mask+Compton” operation, we will separate diffuse and point-sources components in the galactic gamma-radiation with high sensitivity. GECCO will be operating mainly in the pointing mode, focusing on the Galactic Center and other regions of interest. It can be quickly re-pointed to any other region, when alarmed. Expected GECCO performance is as follows: energy resolution <1% at 0.5-5 MeV, angular resolution ∼1 arcmin in the Mask mode (5-7 degree field-of-view, ∼2,000 cm2 effective area), and 3-5 degrees in the Compton mode (~50 degree field-of-view, ∼500 cm2 effective area). The continuum sensitivity is expected to be better than 10−5MeV/cm2/s at 1 MeV. GECCO can be considered for a future NASA Explorer mission.