In this work, a systematic investigation on magnetic critical behavior is performed for the first time on an antiperovskite-type Mn3GaN, which is prepared by intentionally modifying stoichiometry. According to the XRD results, the antiperovskite structure is well preserved, even though all lattice parameters shrink upon reducing Ga and N content down to 60%. The sample exhibits a ferromagneticlike feature with a Curie temperature (T-C) of 394K rather than frustrated behavior in stoichiometric Mn3GaN. Most importantly, the modified Arrott plots, Kouvel-Fisher plots, as well as critical isotherm method self-consistently co-confirm the critical exponents of beta=0.33, gamma=1.23, and delta=4.7, unambiguously indicating that the critical behavior follows the 3D-Ising model around T-C. Published under license by AIP Publishing.