Scintillators based on Ce-doped garnets are regularly co-doped with Mg²⁺ or Ca²⁺ to form Ce ions in 4+ state and reduce undesired afterglow. However overly high Ce⁴⁺ concentration leads to poor light yield performance. In order to understand the reason for variation in luminescence efficiency of Ce³⁺- and Ce⁴⁺-doped garnets we investigate the differences in energy conversion processes in complex LuGd₂Ga₃Al₂O₁₂:Ce³⁺/Ce⁴⁺ ceramics by means of VUV synchrotron irradiation. At first we have established via transmission spectroscopy and X-ray absorption spectroscopy that LuGd₂Ga₃Al₂O₁₂:Ce, Mg sample contains cerium in the 4+ state only. Then we show with VUV spectroscopy efficient interaction of excitons with Gd³⁺ and Ce³⁺, and lack of exciton absorption edge in LuGd₂Ga₃Al₂O₁₂:Ce⁴⁺ excitation spectrum. Instead, Ce⁴⁺ exhibits charge-transfer absorption band in the range of exciton emission. We suggest that when Ce⁴⁺ concentration becomes too high, the exciton → Gd³⁺ → Ce³⁺ energy transfer path is hindered. It leads to high intensity of Gd³⁺ luminescence in Lu₁Gd₂Ga₃Al₂O₁₂:Ce, Mg ceramics, but lowered Ce³⁺ X-ray excited luminescence. Fine balance between 3+ and 4+ Ce concentrations is necessary to achieve the best performance of garnet scintillators.