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Time-domain magnetization dynamics of sputtered GdFeCo (30 nm) amorphous alloy films was measured by pump-probe method using high-power ultra-short pulse fiber laser. The effective g-factor g eff and effective damping constant alphaeff of the GdFeCo films were estimated by using a numerical calculation of Landau-Lifshitz-Gilbert equation. The precessional frequency took a maximum near the magnetization compensation composition C M of the GdFeCo, while the estimated g eff and alphaeff increased around the angular momentum compensation composition C A. The compositional dependences of g eff and alphaeff were roughly described by a mean-field model. The g eff and alphaeff were also estimated from the ferromagnetic resonance (FMR) spectra, and the data from the FMR spectra agreed well with those from the pump-probe measurement except for the composition near C M. The FMR method was unable to excite the magnetization near C M because of the small net magnetization.