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 geff 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 CM of the GdFeCo, while the estimated geff and alphaeff increased around the angular momentum compensation composition CA. The compositional dependences of geff and alphaeff were roughly described by a mean-field model. The geff 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 CM. The FMR method was unable to excite the magnetization near CM because of the small net magnetization.