This study focuses on the wavelength-dependent conductivity of optically-induced 2-dimensional electron gases (2DEGs) in ultra-pure GaN/AlGaN heterostructures grown by Molecular Beam Epitaxy (MBE). Our experiments show that only light with energies larger than the bandgap of GaN (3.4 eV) is able to generate a 2DEG, which is else absent under illumination with wavelengths exceeding 364 nm. The conductivity of the generated 2DEG depends on the number of incident photons with sufficient energy. Moreover, resonant absorption can be clearly observed around the bandgap energy of GaN at room temperature.