Gamma Ray Bursts Are Not That Bright Compared to Other Astrophysical Sources

If aliens were watching our nuclear explosions and they had technology similar to ours, they would not be able to see our explosions from very, very far away. But if you're imagining an alien civilization with the power of our nuclear weapons and they had some sort of cataclysmic event where they blew them all up at once, then you can ask how far away would they be for us to detect it. Our gamma ray sensors are pretty sensitive but remember that things get much dimmer as you get further away.