The electric field of thunderclouds modifies components and energy spectra of the cosmic-ray air shower resulting in an enhancement of gamma-ray fluxes on the ground, known as a gamma-ray glow. The Gamma-Ray Observation of Winter THunderclouds (GROWTH) collaboration has measured this phenomenon for several years in the Hokuriku area of Japan. The present work examines the ambient conditions required to produce spectral features of the previously detected gamma-ray glows. We use Monte Carlo simulations of particle interactions in the atmosphere with the cosmic-ray electron spectrum as input from the EXcel-based Program for calculating Atmospheric Cosmic-ray Spectrum (EXPACS). We focus on three parameters, the strength and length of the electric field and the size of a null-field attenuation region below the electrified region. The average spectrum of observed gamma-ray glows in winter thunderstorms of Japan requires an electric field intensity close to the Relativistic Runaway Electron Avalanche (RREA) threshold of 0.284 MV/m. The vertical size of the electric field region should be in the range of 0.5—1 km. The estimated attenuation region size is 300–500 m, necessary to reduce the low-energy photon flux of the average gamma-ray glows. The whole space of phase capable of generating the gamma-ray glow spectra uses only cosmic-ray electron shower as input. Thus, the interaction between cosmic-ray and the thundercloud is capable of producing the signal detected at the ground.