Zinc Oxide, plays a crucial role in the application of all-optical devices due to its broadband nonlinear optical response. In this work, the broadband nonlinear optical response of Ga-doped ZnO was investigated using femtosecond transient absorption spectroscopy (450-800 nm), revealing ultrahigh modulation depth (~86%) and ultrafast response (~280 fs) at a pump wavelength of 750 nm. Non-degenerate two-photon absorption spectra corrected by dispersion revealed significantly stronger TPA in the visible range. An energy level model based on Ga-related defect explained the enhancement mechanism of TPA and photoluminescence spectrum. This study highlights the significant potential of GZO in future broadband all-optical device applications.