Recently, a radio-fluorogenic gel dosimeter that can evaluate the depth–dose distribution of high-energy carbon ion beams within a radio-therapeutic dose range was reported. This study performed a detailed characterization of this gel dosimeter using five types of ion beams (high-energy carbon, silicon, argon, iron, and xenon ion beams). In addition, this paper explains the mechanism of linear-energy-transfer (LET)-independent sensitivity by evaluating the effect of the composition conditions and contribution of water-decomposed molecules and radicals. Thus, it was observed that the dosimeter could be used to evaluate the dose distribution of various high-energy heavy ion beams in bioequivalent materials. The LET range was 25–8000 eV/nm. Additionally, it was suggested that this feature was caused by two mechanisms, i.e., an increase in the reaction by hydrogen peroxide and a decrease in the contribution of OH radicals by degassing.