Sometimes, there are two heads on the floor, with one belonging to Gibson, and the other at his feet is a partial body that has mostly sunk in to the floor. This can be found out by using console commands to walk through the floor, or attempting to move the head, as the shoulders will come up out of the floor, but the body will not come entirely out of the floor.
In the German gore-censored version of Fallout 3, Gibson's corpse isn't beheaded. However, the other head isn't removed so there are two heads.
Gibson is a reference to Hideo Kojima's video game Snatcher, which starred a minor character named Jean-Jack Gibson who is also found decapitated in the same manner in the ruins of a factory. He too has a note upon him reading "Search the house!" In Snatcher there is also a Gibson's house, wherein his daugher Katrina resides, which also contains a small model home. Within the model are various clues.