GEM solves the electromagnetic gyrokinetic equations which are the appropriate equations for well magnetized plasmas. The plasma is treated statistically as a kinetic distribution function. The distribution function depends on the three-dimensional position, the energy and magnetic moment. The time evolution of the distribution function is described by gyrokinetic theory which simply averages the Vlasov-Maxwell system of equations over the fast gyromotion associated with particles exhibiting cyclotron motion about the magnetic field lines. This eliminates fast time scales associated with the gyromotion and reduces the dimensionality of the problem from six down to five.