Planar Guinier-Preston zones (GP-zones) parallel to a basal plane of an Mg hexagonal lattice are found together with β′ and β₁ precipitates, which have been found in Mg-Gd alloys, in an Mg-2 at%Gd-1 at%Zn (Mg₉₇Gd₂Zn₁) alloy aged at 200°C for 150 hrs. From the combination of high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark field scanning transmission electron microscopy (HAADF-STEM), we can determine the structure of the GP-zone, in which Gd and/or Zn atoms occupy at ordered positions in two close-packed planes sandwiching a close-packed plane of Mg atoms.