We developed a multidimensional GAs Multicomponent Mixture Analysis (GAMMA) code in order to investigate chemical reaction behaviors related to an air ingress accident and the thermofluid transients in high-temperature gas-cooled reactors. The implicit continuous Eulerian technique is adopted for the reduction of a 10N × 10N matrix into an N × N pressure difference matrix and fast transient computation. In the validation with a high-temperature engineering test reactor (HTTR)-simulated air ingress experiment, the onset times of natural convection are accurately predicted within a 10% deviation. Small internal leaks in the HTTR-simulated test facility have been found to significantly affect the consequence of air ingress. In all the simulated cases for a SANA-1 afterheat removal test, the predictions of GAMMA are in a high level of agreement with the measured temperature profiles and are comparable to the results of other codes (TINTE, THERMIX/DIREKT, and TRIO-EF).