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We revisit the reversible magnetocaloric effect of itinerant ferromagnet Mn3GaC near the ferromagnetic to paramag-netic phase transition by adopting the experimental and theoretical methods and critical behavior of Mn-rich Mn3GaC with an enhanced FM interaction. Landau theory model cannot account for temperature dependent magnetic entropy change which is estimated from thermal magnetic methods only considering magnetoelastic coupling and the electron-electron interaction, apart from molecular mean-field model. Critical behavior is studied by adopting the modified Arrott plot, Kouvel-Fisher plot, and critical isotherm analysis. With these critical exponents, experimental data below and above Tc collapse into two universal branches, fulfilling the single scaling equation m= f±(h), where m and h are renormalized magnetization and field. Critical exponents are confirmed by Widom scaling law and just between mean-field model and three-dimensional Heisenberg model, as the evidence for the existence of long-range ferromagnetic interaction. With in-creasing the Mn content, Tc increases monotonously and critical exponents increases accordingly. The exchange distance changes from J(r)～r-4.68 for x=0 to J(r)～r-4.71 for x=0.08, respectively, which suggests the competition of the Mn-Mn direct interaction and the itinerant Mn-C-Mn hybridization. The possible mechanism is proposed.