Honestly, if radiation from a GRB close enough to hurt us hit you, the first thing you'd notice would be intense nausea and/or unexplainable blindness.

And with a prolonged full body dose of radiation that would probably make the Chernobyl Elephant's Foot look like a Rabbit's Foot, your central nervous and immune systems would be overwhelmed by a complete bodily failure and you'd just pass out and never wake up.