We report the excitation of the non-steady-state photo-electromotive force in a monoclinic gallium oxide crystal. The crystal grown in an excess of oxygen is insulating and very transparent for a visible light, but this, however, does not prevent us from the formation of dynamic space-charge gratings and observation of the photo-EMF signal at λ=532 nm. The induced ac current is studied for the cases of zero external electric fields. The dependencies of the signal amplitude versus the frequency of phase modulation, light intensity, spatial frequency are measured. The photoconductivity and diffusion length of electrons are estimated for the chosen light wavelength.