Gadolinium gallium garnet (Gd3Ga5O12: GGG) is expected to have very high shock‐impedance compared with sapphire (Al2O3), etc., and thus to be used as a potential new anvil material in shock reverberation experiments on hydrogen and other low‐Z materials. In this study, the Hugoniot‐measurement experiments were performed using both a powder gun and two‐stage gas guns in the pressure range to 100 GPa by means of the inclined‐mirror method. The HEL stress was measured to be larger than 30 GPa. A kink was observed on the Hugoniot in the pressure range higher than 60 GPa, which might be caused by structural phase transition or decomposition. © 2004 American Institute of Physics