"That's why there aren't any really old civilizations"
The GRBs don''t really explain anything. Because it leads to the question why any civilization would need 1 BILLION years to develop the technology ot withstand a GRB. Maybe it would explain why life hasn't evolved beyond the microbial level. But once a civilization has achieved the Iron Age of technology, such a civilization is likely to achieve space faring status within a thousand years, unless of course they wipe themselves out or get struck by a far more common local extinction level event.
I say an asteroid impact wiping out a nascent civilization is far more likely than a GRB wiping out a civilization just a few hundre years more advanced than ours.