The direct flash is not the main surface problem
Earth’s atmosphere absorbs most gamma rays. The deeper hazard is atmospheric chemistry: intense ionisation can create nitrogen oxides that destroy stratospheric ozone.
Models predict years of elevated UV
NASA-supported atmospheric modelling found that a suitably intense Galactic burst could produce large global ozone depletion lasting years, allowing much more biologically damaging UVB radiation to reach the surface.
The Ordovician idea is a hypothesis
Some researchers have proposed a gamma-ray burst as a possible contributor to the late Ordovician mass extinction because its ecological pattern could fit strong UV stress and cooling. NASA explicitly notes that there is no direct evidence that such a burst triggered that extinction.
Distance and direction matter
Most gamma-ray bursts occur far beyond our Galaxy. A dangerous event would need to be both sufficiently close and oriented so that its narrow high-energy beam intersected Earth.
Research record
NASA — Terrestrial ozone depletion due to a Milky Way gamma-ray burstNASA / peer-reviewed record · open sourceNASA SVS — Gamma-ray bursts may have caused ancient extinctionsNASA · open sourceNASA NTRS — Did a gamma-ray burst initiate the late Ordovician extinction?NASA technical record · open sourceCould a Gamma-Ray Burst Cause a Mass Extinction?
Direct answer: Yes in principle. Atmospheric models show a nearby, correctly aimed gamma-ray burst could cause severe ozone depletion and ecological damage. No known mass extinction has been conclusively tied to one.
Credible physical hazard; unconfirmed historical cause. This is a good example of why mechanism and event attribution must remain separate.