It's possible, but very unlikely, that there's a gamma ray burst in our galaxy focused towards Earth. It has to be withing 5,000 to 8,000 light years of Earth, in a galaxy that is 100,000 light years in diameter. So that's not very likely. They may happen every 5 million years close enough to affect us. We are shielded from the immediate effects by our atmosphere, about ten tons per square meter. So it's mainly things like depletion of the ozone layer and acid rain.
A close gamma ray burst may cause extinctions. Maybe mass extinctions if close enough. But I'm sure humans would survive, would be one of the adaptable specis that would survive, since with our technology we are one of the most adaptable species on Earth, at least of animals. And that's with only stone age technology.
Are Gamma Ray Bursts Dangerous? - Universe Today
See also my Could Anything Make Humans Extinct In The Near Future? for an overview of extinction threats to humans.
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