Gehenna is often translated Hell, which transmits centuries of misunderstanding. The word comes from the hebrew גֵּיא בֶן-הִנֹּם (géy vèn-hinnom — valley of the son of hinnom) which was a real physical place where human sacrifices were performed by burning children, according to Jeremiah 7,31-331
31They [the Judeans] have built the high places of Topheth [Literally: "of the fireplace"] in Ben Hinnom Valley in order to burn their sons and daughters in the fire, a thing I did not command; I never entertained the thought. 32“Therefore, look, the days are coming”—the Lord’s declaration—“when this place will no longer be called Topheth and Ben Hinnom Valley, but Slaughter Valley. Topheth will become a cemetery, because there will be no other burial place. 33The corpses of these people will become food for the birds of the sky and for the wild animals of the land, with no one to scare them away.
You can understand through this scripture all the imagery that comes in people’s mind when hearing the word Gehenna.
In the NT, Jesus and James (3,6) are the only ones using the explicit word Gehenna: please look at Matthew 5,22
22I tell you, everyone who is angry with his brother or sister will be subject to judgement. Whoever insults his brother or sister, will be subject to the court. Whoever says, ‘You fool!’ will be subject to hellfire [Literally: "the Gehenna of fire"]
And a few verse later, in the same line of reasoning: Matthew 5,29-30
…it is better that you lose one of the parts of your body than for your whole body to be thrown into hell [Gehenna]…it is better that you lose one of the parts of your body than for your whole body to go into hell [Gehenna].
In the Sermon on the Mount Jesus implies that not only the soul but also the body can go or be thrown in Gehenna. Going is an active verb when being thrown is passive. It is often assumed that when a passive verb is used, it is God who is being the scene. But nothing makes it obvious, and we will see later on that it may well work an other way. What seems obvious by the way is that being subject [ἔνοχος ἔσται] to the Gehenna of fire is the same as being subject of judgement.
Don’t fear those who kill the body but are not able to kill the soul; rather, fear him who is able to destroy [ἀπολέσαι] both soul and body in hell [Gehenna].
Here again, body and soul are prone to be destroyed. It has two implications: first, it means that contrary to popular greek thinking, souls are not "immortal", and second (in addition with what was said in the Sermon On the Mount) it means that if the body goes into Gehenna it is for its destruction. We’ll talk about that later too.
In this passage it seems that it is not the devil who kill the soul as he is not mentioned anywhere. But in the same manner, as implied in the following verses which urge the disciples not to be afraid of God, it seems that it is not God who will destroy souls. The question though is: "who is it?". This is also something to be explored later!
In Matthew 18,9 during another of his major speeches, Jesus uses again the same image as in the Sermon Of the Mount:
It is better for you to enter life with one eye than to have two eyes and be thrown into hellfire.
In Matthew 23 in the famous rebuke against religious hypocrisy, Jesus uses twice the word Gehenna:
15“Woe to you, scribes and Pharisees, hypocrites! You travel over land and sea to make one convert, and when he becomes one, you make him twice as much a child of hell [Gehenna] as you are!
33“Snakes! Brood of vipers! How can you escape being condemned to hell [Gehenna]?
Being a child of Gehenna seems to be a religious and contagious affection that lead to condemnation to… Gehenna. If it seems to be a funny conclusion, it is an interesting clue about how Gehenna works. We’ll use it later.
In Mark 9, 42-50, Jesus uses three or five times2 the word Gehenna, but it is a parallel of the Sermon on the Mount. And the only time when Jesus uses the word Gehenna in Luke (12,5) is a parallel of Matthew 10,28.
So if Gehenna appears in 11 sentences pronounced by Jesus, considering the synoptic parallelism he talks about it in only 4 occasions in which it is to be feared as prelude to destruction not only of the soul but also of the whole body.
The concept of a place of torments is widespread in the Bible, but it is also expressed by other expressions than Gehenna: for example: "eternal fire"3 is an expression that can replace it (see Matthew 18,8-9). But this eternal fire, may not be eternal as we understand eternal today. You can realize that by reading in Jude 5-7 that:
5...Jesus4 saved a people out of Egypt and later destroyed [or removed, made disappear, lost] those who did not believe; 6and the angels who did not keep their own position but abandoned their proper dwelling, he has kept in eternal chains in deep darkness for the judgement on the great day. 7Likewise, Sodom and Gomorrah and the surrounding towns committed sexual immorality and perversions, and serve as an example by undergoing the punishment of eternal fire.
Are Sodom and Gomorrah still burning? Maybe in a spiritual realm, but we can’t see it. Here eternal fire is synonymous with: disappearing, being lost, or being kept in eternal5 chains in deep darkness for the judgement on the great day. And it seems that these eternal chains will last... until judgement so they eternal may not be forever.
So we do not have a precise description of Gehenna, we just know that it is better to avoid it.
1 See also Jeremiah 19,6.
2 Most manuscripts doesn’t have the v44 nor the v46 that seems to be later additions to emphasize the threat.
3 Greek πυρὸς αἰωνίου.
4 Some manuscripts have: The Lord.
5 The word eternal is not the same as in the previous expression (ἀϊδίοις) but it means the same.