For the Fallout: New Vegas weapon of the same name, see Gehenna (GRA).
The following is based on Van Buren and has not been confirmed by canon sources.
Gehennas are monsters found in Burham Springs. They are slightly larger than humans and appear to be made of black tar and shiny oil. They are vaguely human-shaped, but their features are hard to analyze through the flames that constantly sputter across their bodies. Their eyes are bulbous glowing orange spheres that seem stuck into their ovoid heads. Gehennas are dangerous both for their fiery melee attacks and the toxic fumes they breathe onto enemies. When they die, they collapse into a disgusting pile of smoking black goo. Gehennas are fairly mindless monsters, usually attacking anyone they see who moves too quickly. They have very poor senses, and can sometimes overlook creatures that stand still. They are related to the even more dangerous molechs.
Given both that the Gehennas are constantly on fire and that they used to be human beings, their name is a reference to a valley location outside of Jerusalem of the same name, mentioned in the King James Version of the Hebrew Bible to be where apostate Israelites, as well as followers of various other gods, performed ritual human sacrifice by fire.