In this study we investigate the power-dependent femtosecond transient gain of direct-gap Ge0.91Sn0.09 thin films crystallized on amorphous SiO2 layers. At carrier injection density Δn<1×1017 cm-3, there is no optical bleaching. When Δn increases to ~8×1018 cm-3, the Ge0.91Sn0.09 film becomes transparent at λ~2000 nm. At higher injection of Δn~2×1019 cm-3, net transient gain is observed at λ=1900-2150 nm, with a peak gain of 6300 cm-1 at λ=2000 nm. Further increasing Δn to ~1×1020 cm-3, the peak of transient gain red-shifted to 2100 nm due to the bandgap renormalization, and the peak gain further increases to ~8000 cm-1. The transient gain coefficient of Ge0.91Sn0.09 is similar to III-V direct bandgap semiconductors at similar injection levels. The gain lifetime was measured to be ~0.1 ns at λ=2100 nm and Δn~6×1018 cm-3. These results confirm that direct-gap GeSn crystalized on amorphous SiO2 is a good optical gain medium towards 3D photonic integration.