We discuss high-energy gamma-ray emission from gamma-ray bursts (GRBs). When such high-energy gamma rays are expected, e± pairs may play an important role both inside and outside the source. e± pair-signatures inside the source (the pair-annihilation bump and the cutoff due to the pair-creation process) are useful for diagnosing the fireball of GRBs. The recipes for diagnoses are largely model-independent and allow us to extract information even from the non-detection of either pair-signature. If the pair-creation cutoff energy is high enough, even TeV gamma rays can escape from the source. But such high-energy gamma rays produce e± pairs due to attenuation by the comic background photons, which leads to delayed secondary emission via the inverse-Compton scattering. This delayed emission is useful as a probe of the cosmic infrared background (CIB) and extragalactic magnetic field (EGMF) in the void region. The future-coming Gamma-Ray Large Area Space Telescope (GLAST) satellite will provide us with good chances to observe such pair-signatures and may enable us to obtain valuable information.