GEM uses the delta-f particle-in-cell (PIC) plasma simulation method. An expansion about an adiabatic response is made for electrons to overcome the limit of small time step, which is caused by the fast motion of electrons. GEM uses a novel electromagnetic algorithm allowing direct numerical simulation of the electromagnetic problem at high plasma pressures. GEM uses a two-dimensional domain decomposition (see domain decomposition method) of the grid and particles to obtain good performance on massively parallel computers. A Monte Carlo method is used to model small angle Coulomb collisions.