We report on generation of coherent optical phonon oscillations in 150 mum thick bulk GaN. With photon energy far below the band gap, the generation mechanisms of coherent phonon modes of A(1)(LO), high- and low-frequency E-2 are revealed to be the impulsive stimulated Raman scattering. We find that one among the two degenerate E-2 modes is selectively detected with a proper choice of probe polarization. Dephasing times range from 1.5 to 70 ps for different modes, and phonon-three-photon absorbed carrier interactions are compared between the A(1)(LO) and the E-2 mode.