The CMS experiment has planned to upgrade its muon system with GEM technology referred to as GE1/1, GE2/1 and ME0 for enhancing physics output. The GE1/1 chambers have already been installed in 2020 and are currently being commissioned. A zero-bias data set was taken to measure the radiation background from the GE1/1 demonstrator known as the “Slice Test”. It is known that the background radiation field is mainly composed of low energy neutrons, photons, electrons/positrons and charged hadrons, namely kaons, pions, and protons. Accurate modelling of the backgrounds using simulations is critical, as an estimation of the expected radiation background for the Phase-2 upgrade can only rely on Monte Carlo-based simulation. In this study, the prediction of radiation background using FLUKA and GEANT4 simulation for the “Slice Test” configuration of the GE1/1 detector is compared with the data.