Finally able to start catching up on some of these older posts after the macro-topical work of the past two months! Yeek! (sorry…)
By the way, this may have been mentioned earlier, but with a little persistence it’s possible to read Gregory’s book for free on Amazon by using the Search Inside The Book feature. (This would of course leave some pages in your browser’s history, but so does reading reviews of his book. 
Plus the browser history can be purged under most systems pretty easily.)
I think James has outlined fairly well how the Valley of Hinnom became an analogical image in Judaism (and for Jesus) as the punishment of sinners either in hades/sheol or after the resurrection of the evil. The imagery is literal because it was in fact a literal place where epic-level sinners once burned things, and where garbage still was being burned in Jesus’ day.
(As an aside, when Jesus is standing on the wing of the Temple during the Synoptic Temptation scenes: that would have been right over the valley of Hinnom!–the farthest drop. It’s also where a priest would watch for the first hint of sunrise over the far-distant Hermon range of mountains, which is typically understood to be the scene for the phase of the temptation involving seeing all the nations of the world in a moment.)
Aside from the Matt 10 and Mark 9 refs previously mentioned, the term (Ge’hinnom, or ravine of Hinnom, transliterated over to Greek) occurs in the NT in the following places: Matt 5:22, 29-30; 18:9; 23:15,33; Luke 12:5; James 3:6.
James 3:6 “Look here!–what amount of fire is kindling what amount of material! And the tongue is a fire, a world of injustice. The tongue is constituted among our members as that which is spotting the whole body, and aflaming the wheel of our lineage, and is aflaming by Gehenna.” (I strongly suspect this has underlying thematic connections back to Isaiah 6:5-7, by the way.)
Matt 5:21-30: "You have heard that it was said to the ancients ‘MURDER NOT’, and ‘Whoever murders, shall be guilty before the judging’. But I am saying to you, that everyone who is angry with his brother shall be guilty before the judging; and whoever calls his brother ‘Good-for-nothing!’ shall be guilty before the Sanhedrin; and whoever even says ‘You fool!’ shall be guilty to the Gehenna of fire! {…}
“You have (also) heard that it was said ‘ADULTER NOT’. But I am telling you, every man who looks upon a woman to lust for her, has committed adultery with her already in his heart! Now, if your right eye makes you stumble, tear it out and throw it away!–for it is better for you that part of your body should die and you should enter the kingdom of God with only one eye, than to be cast with two eyes into the flaming garbage dump! And if your foot is making you stumble, cut it off!–for it is better to enter life as lame, than to be thrown with both feet into the flaming garbage dump! And if your right hand causes you to stumble, cut it off and throw it away!–for it is better for you that part of your body should die and you enter the life of the kingdom of God as a cripple, than for your whole body to be cast with both hands into the flaming garbage dump!” (I translated {gehenna} as ‘flaming garbage dump’ for contextual purposes.)
There is a repeat of this precept to the disciples during the final scene in Capernaum, which is paralleled in Matt 18 and Mark 9. (Luke also has the sequence of events, but omits this particular scene.) A harmonization of the relevant verses would read:
"Look here, then!–you should not be despising one of these little ones; for I am telling you: their angels in the heavens look continually on the face of My Father Who is in the heavens!
"But whoever is causing one of these little ones who believe in Me to stumble… it would be better for him to hang a ton of stone around his neck and be sunk in the open sea!!
"Woe to the world because of stumbling blocks! For, it is inevitable that stumbling blocks will come–but woe to the one through whom the stumbling block is coming!
"Now if your hand or foot is causing you to stumble, cut it off!–for it is better to enter life as lame, than to be thrown with both your hands and feet into the flaming garbage dump!
"And if your right eye makes you stumble, tear it out and throw it away!–for it is better for you that part of your body should die and you should enter the kingdom of God with only one eye, than to be cast with two eyes into the flaming garbage dump, where (as it is written in Isaiah the prophet) ‘Their worm does not die, and the fire is not quenched!’
“But how does that seem to you?! If any man comes to have a hundred sheep, and even one of those happens to be led astray, does he not leave the ninety-nine on the mountains and go and search for the one that is straying?
“And if it turns out that he finds it, I am telling you truly, that he is rejoicing more over this one than over the ninety-and-nine which haven’t gone astray!
"Thus it is not the Will before your Father in the heavens, that even one of these little ones shall be dying!”
"For everyone will be salted with fire!! (that is, per the Follower’s account, the fire in Gehenna)
"You are the salt of the earth; and salt is ideal! But if the salt becomes unsalty, how will it be made salty again? With what will you be seasoning it!? It is good for nothing anymore, except to be thrown out and trampled underfoot by men! Whoever has ears to hear, let him hear:
"Have salt in yourselves; and be at peace with one another.”
The Luke 12 context occurs while Jesus is on the final approach to Jerusalem from Jericho. The relevant verses read:
"Now, there is nothing covered up that will not be revealed, and hidden that will not be known! Instead, whatever you say in the dark shall be heard in the light, and whatever you speak in the ear in the inner rooms shall be heralded on the housetops!
"But I am saying to you, My friends–do not be afraid of those who kill the body, yet after this they have no worse that they can do to you. Now I will tell you as a friend whom you should fear: be afraid of Him Who after killing has authority to cast you in the flaming garbage dump! Certainly I tell you, be afraid of this One!
"Five sparrows are sold for two copper coins, aren’t they? Yet not one of them is forgotten in God’s sight! And even the hairs of your head have all been numbered!
“So do not fear!–you are worth more than many sparrows!”
The final occurrences in the NT are reported in the GosMatt version of the final condemnation by Jesus of the Pharisees and scribes and chief priests before He leaves the Temple:
"Wailing to you, you scribes and Pharisees, you hypocrites!–because you travel around on sea and land to make one convert; and when he does become one, then you make him twice as much a son of flaming garbage as yourselves!! {…}
"Wailing to you, you scribes and Pharisees, you hypocrites!–for you build the prophets’ tombs, and decorate memorials of the fair ones, and say, ‘If we had been in the days of our fathers, we would not have been their partners in the blood of the prophets!’
“So, against yourselves, you give testimony that you are descendants of the murderers of prophets, and fill up the measure of your fathers!–you serpents, you sons of vipers! How shall you escape the judgment of fiery waste!!?”
(Here references to Gehenna are being used within context of rhetorical colloquialism, so I’ve paraphrased the translation accordingly.)
Each occurrence has elements of non-literal metaphor and/or analogy strongly attached to it; but the threat to body as well as soul, by its character, has to have some level of literality about it. The danger isn’t to be waved off, in any case, on grounds of it being a ‘metaphorical’ description. When Jesus gets piççed off, it’s long past time to take Him seriously. 
Luke 12’s ref to Gehenna, as noted above, contrasts that we should not fear those who can kill the body and then do nothing more, compared to the One Who after killing has authority to throw both body and soul into Gehenna. If one is simply dead at death of the body, then there would be no rhetorical point to not fearing one but fearing the other. (Note: the Matthean parallel at Matt 10:26-28ff doesn’t include the qualifying detail that this happens after this One has already killed.)
Yet again, unless there is supposed to be more than one everlasting fire of Gehenna into which sinners are thrown, this topically has to be the same lake of fire as that at the end of RevJohn 20, where resurrected sinners are thrown (along with hades and death) into the lake of fire. Which isn’t the end of the story, even in RevJohn! But still, there’s the body-and-soul-of-sinners-into-fire after the general resurrection.
My extended critique of a popular scholarly apologist for annihilationism, Dr. Samuele Bacchiocchi (which can be found here in our forums) covers a lot of the ground you’re asking about, including the ref to Isaiah and the bodies in the valley.
so I have been reading interviews and the limited access to the book that I can online. I read an interview yesterday where he seemed to suggest that Gehenna was the Lake of Fire or hell if you will. I personally don’t see any scriptural proof that Gehenna was anything other than a literal place with a literal fire, worms, and even a gnashing of teeth. I would love to know at what point in time did Gehenna become symbolic for hell. I also find it interesting that, at least a certain sect of Jews believed that Gehenna was temporary, nine months comes to mind. I would also like to know which scripture you folks best indicates that Gehenna is Hell?
