Simply discussing GRBs as possible causes of Mass extinctions across the galaxy particularly in the distant past does not imply a link with Earth's specific MEs which have multiple postulated causes with different ones more likely suspects in one event than another.

It's virtually certain that the K/T event was impact induced.

Global warming (caused by land mass arrangement changes but the same issue so fondly ignored today) is the chief suspect in the worst event with a small bit of evidence pointing towards impact as an alternative.

From a microbial standpoint the worst ever ME was cause by life itself generating an oxygen atmosphere and destroying most of the habitat for most of Earth's previous inhabitants.

I've never heard anyone postulate a GRB caused any of Earth's ME's so judging the potential consequences of a GRB based on our history of ME's doesn't sound reasonable to me.