Optical-isothermal capacitance transient spectroscopy (O-ICTS) is used to characterize the mid-gap states in n-type GaN grown by hydride vapor phase epitaxy and metalorganic vapor phase epitaxy. Deep levels are resolved by the emission rate of carries in addition to the magnitude of the capacitance transient as a function of incident photon energy. The mid-gap region is dominated by a distribution of traps from which carriers photoionize to the conduction band in the range of about 1.5 to 2.5eV; these traps are suspected of participating in the frequently observed yellow luminescence. The concentration of these mid-gap states are found to be lowest in thick HVPE-grown GaN films.