We report Raman studies of the A1 (TO), E1 (TO), E22, A1 (LO), and E1 (LO) symmetry phonons of GaN from 20 to 325 K. By applying anharmonic decay theory to the observed temperature dependences of the phonon energies and linewidths, we determine the phonon decay mechanisms of these zone-center vibrations. Thermal expansion is taken into account using published temperature-dependent coefficients. The A1 (TO) and E1 (TO) vibrations are described by symmetric two-phonon decay. The E22 decays via the creation of three phonons. Both A1 (LO) and E1 (LO) bands are interpreted by an asymmetric two-phonon decay, with a minor contribution to the decay of the former from the three-phonon creation. Phonon lifetimes are obtained based on the observed linewidths.