The crystalline structures and magnetic and thermodynamic properties of a Gd4Ga2O9 single crystal grown with the optical floating zone technique have been investigated. Gd4Ga2O9 crystallizes in a monoclinic structure with the space group P21/c at room temperature. Temperature-dependent magnetic susceptibility measurements along the three crystallographic axes reveal a paramagnetic (PM) behavior between 2 and 300 K. A Curie–Weiss (CW) law fit was carried out and the CW temperature CW and magnetic frustration parameter f were calculated; these suggest antiferromagnetic (AFM) interactions between Gd3+ spins and a strong magnetic frustration. The field dependence of the magnetization at 2 K further confirms the magnetic frustration characteristics. A distinct -shaped peak at 1.4 K in the heat capacity curves suggests a transition from the PM to AFM phase. The magnetic entropy is contributed solely by Gd3+ ions.