Gehenna originated from the place just outside of Jerusalem that was used as the city dump, hence the references to the worm not dying and the fire not being quenched. The people would have been intimately familiar with it as a place where their trash was sent. People would labor day and night trying to burn up as much as possible, as maggots contunually spread among the filth.
(originally Ge bene Hinnom; i.e., “the valley of the sons of Hinnom”), a deep, narrow glen to the south of Jerusalem, where the idolatrous Jews offered their children in sacrifice to Molech (2Ch_28:3; 2Ch_33:6; Jer_7:31; Jer_19:2-6). This valley afterwards became the common receptacle for all the refuse of the city. Here the dead bodies of animals and of criminals, and all kinds of filth, were cast and consumed by fire kept always burning. It thus in process of time became the image of the place of everlasting destruction. (M.G. Easton M.A., D.D., Illustrated Bible Dictionary, 1897)
As you can see it was a very potent image for the Jews of the day.