Eu3+-or Ce3+-doped gadolinium gallium aluminum garnet (GGAG), Gd3Ga2Al3O12, phosphors are fabricated using solid-state reactions with Gd2O3, Ga2O3, Al2O3, CeO2 and Eu2O3 powders. The Eu3+-or Ce3+-doped Gd3Ga2Al3O12 phosphors are sintered at 1300 °C or 1600 °C for 5 hours by using an electric furnace under normal atmosphere. X-ray diffraction and field-emission scanning electron microscopy studies are carried out in order to analyze the physical properties of these materials, and their luminescence properties are also measured by using UV and X-ray sources. The Eu3+-or Ce3+-doped Gd3Ga2Al3O12 phosphors show higher light yields in comparison to commercial phosphors such as Gd2O2S:Tb (gadox). This indicates that Gd3Ga2Al3O12:Eu3+ phosphors are promising materials for use in X-ray imaging and dose monitoring at proton beamlines.