The optical gain spectra of strongly excited multiple GaAs-(Ga,Al)As quantum wells have been determined using the variable stripe length method. The gain spectra are obtained by analysing the dependence of the amplified luminescence intensities on stripe length. In the novel fitting procedure we applied it is assumed that the optically generated carriers of the electron-hole plasma are in quasi equilibrium. It is found that this assumption is justified in all the cases considered here where saturation effects are absent.