The signature of the strong-coupling regime is unambiguously evidenced in a GaN-based microcavity (MC) above the polariton lasing threshold P-thr at room temperature through the observation of the upper polariton branch. The MC system exhibits a renormalization of the polariton dispersion curve, namely, a reduced normal-mode splitting compared to the low-density regime. Next the dependence of P-thr as a function of exciton-photon detuning is investigated in the 4-340 K temperature range, which allows accessing the polariton lasing phase diagram. The observation of polariton lasing over such a broad range of temperatures reveals a clear transition from a kinetic to a thermodynamic regime with increasing temperature.