A hot carrier relaxation dynamics are studied in both n-type and p-type GaN films grown on sapphire by molecular beam epitaxy. A novel femtosecond pump-probe technique is used in which the carriers are excited by an infrared pump and the carrier dynamics are monitored by a tunable near UV probe. Complex transients, showing bleaching and induced absorption, are observed in both samples. The electron dynamics are fitted by a model in which the LO-phonon emission is the dominant energy relaxation process. The hole dynamics also have been discussed and compared to the electron dynamics. Hot phonon effects seem play a major role to reduce the effective strength of the carrier-LO phonon scattering rate in both cases.