A basic ground plate adapter which is adapted for utilization with wiring devices installable in non-metallic electrical boxes, and in particular, a ground plate adapter which is employable for a single-gang and is selectively connectable to ground plate adapters for constructing multi-gang arrangements as may be required for specific physical and utilitarian applications. The basic ground plate adapter includes a generally inverted U-shaped bracket structure adapted to clampingly engage the edge or rim of the sidewall of a non-metallic or plastic electrical box, and includes a Z-shaped plate arrangement or further bracket attached to the inverted U-shaped bracket structure. The ground plate adapter also includes a threaded aperture for receiving a ground screw. The ganging ground plate adapter, which in overall structural design is similar to the basic ground plate adapter which is normally used for single-gang installations, incorporates a laterally extending arm member having a depending bracket structure at its free or distal end, including a bracket having an aperture formed therein and adapted to be superimposed on or connected to the Z-shaped bracket on the basic ground plate adapter so as to form an interconnected or ganged two-bracket ground plate construction for a two-gang utilization.