Just die.

That's not actually a problem, because dangerous GRBs are simply not that common. Earth gets hit by one every day, but, that includes GRBs from very distant galaxies that are so weak when they hit us that they would not be any danger even to a completely unshielded person in orbit. GRBs of sufficient power to be an actual threat to people in a tin can is one/millions of years, at which point it's a propulsion problem again.

Yes, there are lots of things that could kill the ship on route. No, none of those are actually a significant problem compared to the basic "how to get there before everything breaks down from internal issues".