We present the first comprehensive investigation of the bulk properties, both optical and structural, of cubic GaN as grown by plasma-assisted molecular-beam epitaxy on vicinal (100) GaAs substrates. X-ray measurements determined the crystal structure of GaN/GaAs to be cubic with a lattice constant of 4.5 Å. High resolution transmission electron microscopy revealed a high density of planar defects propagating along the GaN {111} planes. The majority of the defects originated from disordered regions at the GaN/GaAs interface. The optical properties of the films were investigated by cathodoluminescence which revealed a broad midgap peak as well as several sharp emission peaks just below the expected band gap. The data imply that the room temperature band gap of cubic GaN is approximately 3.45 eV.