This paper presents a study on reflection-free one-way edge modes in a gyromagnetic photonic crystal. The authors show that electromagnetic one-way edge modes, similar to quantum Hall edge states, can exist in gyromagnetic photonic crystals without requiring Dirac points or gyroelectric materials. These modes are determined by the Chern number of the photonic bands, which is non-zero for all bands below a gap. The study uses a square-lattice gyromagnetic Yttrium-Iron-Garnet (YIG) photonic crystal operating at microwave frequencies, which lacks Dirac points but has strong time-reversal breaking. The edge modes occupy a 10% band gap and are immune to backscattering, making them suitable for applications such as slow-light structures and microwave isolators. The authors demonstrate that these edge modes can be mapped to electronic wavefunctions in a quantum Hall system, and that the Chern number determines the number of edge modes. The study also shows that these edge modes can be used to create a one-way waveguide that circumvents defects, with 100% transmission across strong defects. The results are supported by numerical simulations and are applicable to both 2D and 3D structures. The study highlights the potential of gyromagnetic photonic crystals for applications in optical and microwave technologies.This paper presents a study on reflection-free one-way edge modes in a gyromagnetic photonic crystal. The authors show that electromagnetic one-way edge modes, similar to quantum Hall edge states, can exist in gyromagnetic photonic crystals without requiring Dirac points or gyroelectric materials. These modes are determined by the Chern number of the photonic bands, which is non-zero for all bands below a gap. The study uses a square-lattice gyromagnetic Yttrium-Iron-Garnet (YIG) photonic crystal operating at microwave frequencies, which lacks Dirac points but has strong time-reversal breaking. The edge modes occupy a 10% band gap and are immune to backscattering, making them suitable for applications such as slow-light structures and microwave isolators. The authors demonstrate that these edge modes can be mapped to electronic wavefunctions in a quantum Hall system, and that the Chern number determines the number of edge modes. The study also shows that these edge modes can be used to create a one-way waveguide that circumvents defects, with 100% transmission across strong defects. The results are supported by numerical simulations and are applicable to both 2D and 3D structures. The study highlights the potential of gyromagnetic photonic crystals for applications in optical and microwave technologies.