We have investigated the effect of light Si-doping on the optical properties of high quality GaN films with the method of low temperature photoluminescence. It is found that the peak (I_x) at 3.473 eV always appears in the photoluminescence spectra of lightly Si-doped GaN. The relative intensity of peak I_x to heavy-hole free exciton peak increases linearly with the increasing concentration of Si doping, providing a strong support to the assignment that I_x originates from inelastic scattering of free excitons by Si donors. In addition, a rarely reported peak (P_x) at 3.365 eV can only be clearly observed in the PL spectrum of unintentionally doped GaN sample. Its intensity is found to reduce dramatically with the decrease of residual carbon concentration based on the secondary ion mass spectrometry analysis. P_x is attributed to the excitons bound to carbon-related complex defects.