A GRB wouldn’t cause much immediate death. The burst itself lasts from seconds to a minute and would be sufficiently blocked by the atmosphere that whatever reached the surface would not be dangerous.

The danger to life comes later. The burst interacts with the oxygen and nitrogen in the atmosphere in a way that would greatly reduce the ozone layer and produce a photochemical smog.

This would give us years of dangerous UV levels, a global cooling effect that might not be too bad but could be very bad if we happen to be a time of climate instability, and acid rain.