<p>This study focuses on the numerical modeling of wave propagation in gain media, a critical element in laser physics that amplifies electromagnetic fields through electron interactions. The gain process is represented by a four-level atomic model combining Maxwell’s equations with nonlinear Ordinary Differential Equations to describe the evolution of electronic populations. While the Finite Difference Time Domain method has traditionally dominated this domain, this work introduces a high-order Discontinuous Galerkin Time-Domain method, tailored for solving such complex systems in 3D. This method is implemented with a second-order Leap-Frog temporal scheme and includes approximations for nonlinear terms. Stability is proven via energy estimates, both for the continuous problem and its discrete counterpart. Numerical validation, carried out on a 3D cubic cavity using manufactured solutions, confirms the accuracy and convergence of the method, followed by a detailed investigation of a three-dimensional physical case. The work highlights the potential of Discontinuous Galerkin Time-Domain methods to enhance the precision and flexibility of numerical simulations in laser physics.</p>