Gibson's scrap of paper
It is found on Gibson's corpse in the Capitol Post office building in L'Enfant. Gibson's house is located in Minefield. You must have the key (which can also be found on Arkansas) to unlock the door. Inside you'll find a model of home (Very Hard lock) on a small table near the door and by the TV, that can also be unlocked with the key.
Search the House!
Gibson's scrap of paper appears in Fallout 3.