That depends on the orientation with regard to the poles. If the source were directly over a pole, only fifty percent of the planet would be directly exposed. If it were over the equator, however, the whole planet would be, if it were a magic GRB that lasted way longer than usual.
Just how many nines reduction from peak intensity after a few minutes are we talking? Because I'd imagine 0.01% of a nearby gamma ray burst would still be bad news.