  Analysis is performed on ultra-high resolution large-scale cosmological
radiation-hydrodynamic simulations to, for the first time, quantify the
physical environment of long-duration gamma-ray bursts (GRBs) at the epoch of
reionization. We find that, on parsec scales, 13% of GRBs remain in high
density ($\ge 10^4$cm$^{-3}$) low-temperature star-forming regions, whereas 87%
of GRBs occur in low-density ($\sim 10^{-2.5}$cm$^{-3}$) high temperature
regions heated by supernovae. More importantly, the spectral properties of GRB
afterglows, such as the neutral hydrogen column density, total hydrogen column
density, dust column density, gas temperature and metallicity of intervening
absorbers, vary strongly from sightline to sightline. Although our model
explains extant limited observationally inferred values with respect to
circumburst density, metallicity, column density and dust properties, a
substantially larger sample of high-z GRB afterglows would be required to
facilitate a statistically solid test of the model. Our findings indicate that
any attempt to infer the physical properties (such as metallicity) of the
interstellar medium of the host galaxy based on a very small number of (usually
one) sightlines would be precarious. Utilizing high-z GRBs to probe
interstellar medium and intergalactic medium should be undertaken properly
taking into consideration the physical diversities of the interstellar medium.
