We propose an emission mechanism of prompt gamma-ray bursts (GRBs) that can reproduce the observed nonthermal spectra with high radiative efficiencies, >50%. We show that, if e± pairs outnumber protons, a radiation-dominated photosphere becomes unstable to the phase separation between pairs and proton clumps. Pairs are continuously shock heated by proton clumps, scattering the thermal photons into the observed broken power-law shape. If GRBs come from pair photospheres, pair annihilation lines are predicted above continua. A closure relation exists between the pair annihilation line and the pair creation cutoff, which enables GLAST to test the model and also constrain physical quantities such as the Lorentz factor, optical depth and pair-to-baryon ratio, only from observables, even without either line or cutoff. We also discuss the Suzaku/WAM results of the time-resolved Ep-L (Yonetoku) relation and the Ep-L relation for short GRBs. We suggest that the short GRBs could arise from the self-created photospheres. We also deal with the hypernova and GRB remnants emitting TeV gamma-rays via the decay of accelerated radioactive isotopes, such as 56Co conveyed by the GRB jets.