Shock acceleration mechanism in gamma-ray bursts may produce photons up to a few PeV energy. Photons above 10-100 GeV and below a PeV energy are trapped inside the fireball due to a high opacity of electron-positron pair production with other photons. High energy photons escaping the fireball may interact with cosmic background radiation and provide delayed gamma-ray emission detectable by GLAST. Detection of the prompt and delayed emission may provide useful constraints on the gamma-ray burst model and also on the inter-galactic magnetic field.