The safe control and dissipation of Runaway Electrons (REs) generated in tokamak plasmas is vital for the operation of future fusion reactors. Measuring the evolution of RE energy in tokamaks is important for understanding their generation, transport, and termination. A new gamma ray spectrometer using a 2″ × 2″ cylindrical, cerium doped lanthanum bromide (LaBr3:Ce) scintillator coupled to a fast photomultiplier tube was developed for studying runaway electrons on the Tokamak à Configuration Variable (TCV). This diagnostic is capable of measuring the RE bremsstrahlung energy spectrum between 1 and 20 MeV with a peak height resolution of 1.91% at 1.44 MeV. A calibration of the detector using its self-emission up to 2.66 MeV was performed and modeled using Geant4. This model was then applied to predict the detector response to photons originating from TCV. The diagnostic was employed to measure the high energy Hard X-Ray (HXR) spectrum with energies in the range from 0.5 MeV to ∼8MeV on TCV for the first time. In TCV, the observations made with this system are demonstrated to provide insights into the change in RE energies during current ramps.