This paper focuses on the acquisition and processing of gravity data collected in the Canton of Geneva in the framework of the InnoSuisse funded project GECOS (Geothermal Energy Chance of Success). The goals of GECOS is to reduce the costs and the exploration risk for geothermal exploration by integrating high resolution data acquisitions such as gravity, S-waves reflection seismic and 3D DAS VSP (Distributed Acoustic Sensing Vertical Seismic Profiling). The main geological challenges in geothermal exploration in the Geneva area are the characterization of the lithological heterogeneities and the fault zones affecting potential geothermal targets in the Quaternary sediments, Oligecene Molasse sequence and the Mesozoic Units. The study area covers the central part of the Canton of Geneva and overlaps with the location of two geothermal exploration wells drilled by SIG in the framework of the Geothermie 2020 program. A total of 1714 new stations were collected in 71 days of field work thanks to the collaboration between the University of Geneva and GEO2X SA. The goal of the survey has been to constrain the lateral density variations associated to lithological anisotropies in the Quaternary, Oligecene Molasse and in the Mesozoic carbonate sequence. The results of the survey showed that the new stations produced a dramatic increase in resolution compared to the gravity data available at the regional scale. In particular Quaternary deposits are much better constrained than before, which is a crucial step forward as these sediments are known to be lithologically and geometrically heterogeneous and locally can host hydrocarbon gas pockets, which can represent an element of risk for drilling operations. 3D inversion processing allowed producing a realistic 3D density model down to about 1000m in depth, where the main present-day geothermal targets are located, proving that gravity can be a powerful tool for prospection and possibly for time-lapse monitoring of production.