Gd₄Co₂Mg₃ (Nd₄Co₂Mg₃ type; space group P2/m; a=754.0(4), b=374.1(1), c=822.5(3) pm and β=109.65(4)° as unit cell parameters) was synthesized from the elements by induction melting in a sealed tantalum tube. Its investigation by electrical resistivity, magnetization and specific heat measurements reveals an antiferromagnetic ordering at T_N=75(1) K. Moreover, this ternary compound presents a metamagnetic transition at low critical magnetic field (H_cr=0.93(2) T at 6 K) and exhibits a magnetic moment of 6.3(1) μ_B per Gd-atom at 6 K and H=4.6 T. Due to this transition the compound shows a moderate magnetocaloric effect; at 77 K the maximum of the magnetic entropy change is ΔS_M=−10.3(2) J/kg K for a field change of 0-4.6 T. This effect is compared to that reported previously for compounds exhibiting a magnetic transition in the same temperature range.