Terahertz amplification and lasing regimes in open plane-parallel metal-dielectric resonator with population inverted (active) graphene are studied theoretically. Terahertz lasing arises at the Fabry–Perot resonance in the open-side metal-dielectric substrate of graphene in the vicinity of the reststrahlen band edge of GaAs superstrate due to photon-phonon interaction in GaAs.