The character of the magnetic phase transitions in the metallic antiperovskite Mn 3 GaC 1− δ (δ=0, 0.22) was analyzed to better evaluate the associated magnetic entropy changes under various field and temperature conditions. The stoichiometric Mn 3 GaC compound exhibits an abrupt magnetostructural transition at T=159K that produces a large entropy change ΔSM with a plateau-like temperature dependence. A unique spike in ΔSM of Mn 3 GaC at T=156.5K, associated with the abrupt magnetic transformation, is attributed to the interplay between the temperature- and field-induced transitions. The peak magnitude of this entropy change spike approaches 33J/kgK, over two times the averaged ΔSM. It was observed that the field development of ΔSM mimics the analogous magnetization behavior. An analytical phenomenological model based on the temperature and field dependence of the magnetization of this type of magnetic transition was developed to account for these results.