Meaning that as a layman, I don't see how we'd distinguish between a past where Earth was hit by a "lethal" GRB 2 billion years ago (when there were just eukaryotes, procaryotes and cyanobacteria), and one were it wasn't.

Indeed, according to Wikipedia at least, we don't know whether the Ordovician–Silurian extinction event was caused by a GRB or not.