Gargantuavis is known from three or possibly four bones: a synsacrum (the fused vertebrae above the hip), a partial illia (hip bone), and a partial femur (upper leg bone), which was referred to the species based on the fact that it seems to fit well with the hip. A second partial synsacrum similar to the type specimen had been described in 1995, and may also belong to Gargantuavis.