Not OP, but yes, I've read the one+ gospel you're referring to. (If I remember correctly, the gospels of Matthew and Luke contain some ideas that were probably copied from Mark; this sounds like one of them.) Personally, I think the narrative of Matthew was carefully crafted--some might even say embellished--to persuade Jews to accept and spread Jesus's teachings. Therefore, it doesn't surprise me that it would include references to a cursed valley mentioned in the Old Testament, where bodies were burned and (legend has it) children were sacrificed to false gods. The Wikipedia article on Gehenna is pretty interesting.