Nathanial Gibbons was a member of the Gibbons family. He was supposed to take his little sister Eliza Gibbons to a Halloween party the day the bombs fell, but he wasn't there to do so. Eliza mentions in her journal that lately Nathanial was gone a lot  so it was a common occurance for him to dissapear. In his stead his mother Victoria Gibbons took Eliza to the party. As such, all three of them were outside when the bombs fell.
The other family members, his grandfather Patrick Gibbons, his father Frederick Gibbons, and his brother Steven Gibbons, locked themselves in the family bunker where they became trapped after the machinery controlling the door was broken.