In this work, a novel parameter called the surface skirt (Ss) is introduced. The electron beam scattering by gaseous environment is the fundamental parameter limiting the performance of the Gaseous Scanning Electron Microscopy (GSEM). The result is the enlargement of the primary beam characterized by the radius skirt Rs. The scattering phenomena require a much closer re-examination. In fact, depending on the localization of EDX detector and the particles shape to analyze, the collected signal after the beam skirt will be different and Rs also will be different. So, except for homogeneous materials, Rs can not describe the scattering behavior. In this study, the surface skirt Ss instead of the radius skirt is introduced. Unlike Rs, the results show that Ss is a linear function versus pressure. This may help to use Ss in different scattering regimes and for a best interpretation of the consequences of electron scattering beam by gaseous environment. For demonstration, two gases environment: helium and water vapor are given but the results are valid whatever the environment used.