First let me say that I do not like using the term "Hell" and have not for the longest time in regard to this topic. The etymology of the word goes back to the 1611 English and is a correct translation of the word "Hades". Hell originally meant "a hole in the ground" and was commonly used to refer to a place where they would store potatoes. It is a word that is tied better to Hades and only came to be associated with eternal punishment because the KJV translators and the world at large did not differentiate doctrinally between Hades and Gehenna the same way there was no doctrinal distinction made between Heaven and Paradise (though the translators at least made a word distinction).
Hades, as per Luke 16, is the place that souls would go when a person died. It is divided into two parts, Paradise and Torments. It is to Paradise that Jesus went with the thief on the cross after their deaths. It is in Paradise that Lazarus the beggar found comfort. In Hades, there is a chasm or gulf that separates the two portions of Paradise and Torments. Torments is for those who were not faithful to God in life such as the rich man in Luke 16. Hades is not the place of eternal damnation. It is a distinct concept from Gehenna.
Gehenna, in every passage I see, is a different place. It is initially a reference to the valley of Hinnom where children were sacrificed to Moloch before Josiah put a stop to it (Joshua 15:8; 18:16; 2 Kings 23:10; Jer. 7:30-33). Later, criminals were buried there and garbage was burned there. In the NT, every reference to Gehenna is from Jesus except the last:
Mat 5:22 - But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of Gehenna fire.
Mat 5:29 - And if thy right eye offend thee, pluck it out, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into Gehenna.
Mat 5:30 - And if thy right hand offend thee, cut it off, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into Gehenna.
Mat 10:28 - And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in Gehenna.
Mat 18:9 - And if thine eye offend thee, pluck it out, and cast it from thee: it is better for thee to enter into life with one eye, rather than having two eyes to be cast into Gehenna fire.
Mat 23:15 - Woe unto you, scribes and Pharisees, hypocrites! for ye compass sea and land to make one proselyte, and when he is made, ye make him twofold more the child of Gehenna than yourselves.
Mat 23:33 - Ye serpents, ye generation of vipers, how can ye escape the damnation of Gehenna?
Mar 9:43 - And if thy hand offend thee, cut it off: it is better for thee to enter into life maimed, than having two hands to go into Gehenna, into the fire that never shall be quenched:
Mar 9:45 - And if thy foot offend thee, cut it off: it is better for thee to enter halt into life, than having two feet to be cast into Gehenna, into the fire that never shall be quenched:
Mar 9:47 - And if thine eye offend thee, pluck it out: it is better for thee to enter into the kingdom of God with one eye, than having two eyes to be cast into Gehenna fire:
Luk 12:5 - But I will forewarn you whom ye shall fear: Fear him, which after he hath killed hath power to cast into Gehenna; yea, I say unto you, Fear him.
Jam 3:6 - And the tongue is a fire, a world of iniquity: so is the tongue among our members, that it defileth the whole body, and setteth on fire the course of nature; and it is set on fire of Gehenna.
3 of these verses give us no indication of the nature of Gehenna other than it is a place nobody should want to go.
4 of these verses use the term fire in association with Gehenna. Mark speaks of this fire as a fire that never shall be quenched. That is a critical point. I'll come back to it with more verses in a minute.
Matt. 23:15 uses the term child of Gehenna which I believe is just a reference to the mortality, weakness, and ultimate destination of those Jesus was speaking to. It's not that they were offspring of Gehenna.
Matt. 23:33 uses the term damnation of Gehenna. The word damnation is krisis which I believe we get our word "crisis" from. The first meaning of the word in the Greek is separation or sundering, especially through the result of a "legal" trial. It can mean judgment, condemnation, sentence for wrong doing, or danger.
Matt. 10:28 is the only passage that uses the term Gehenna where a word associated with it is translated "destroy". The Greek word is apollymi and it can be translated destroy, but the idea behind it is to put an end to, to put out of the way, to render useless, to kill or to put to death as in sentencing someone who has received a judgment, to perish or be lost. If we use this word in conjunction with krisis, then the destruction is in terms of complete putting away from God, eternal separation from God, which we know is spiritual death (Isa. 59:1-2).
With regard, then, to Mark and the unquenchable fire. We also have the concept of "everlasting fire" in the parallel passage of Matt. 18:8. Therefore, everlasting is a word of consequence.
Throughout the OT, God is referred to as "everlasting". It is the Hebrew word 'owlam. We understand then, that this is not a concept of time, but beyond time...eternal. We see this again in the NT in the word aionios. It is spoken of in reference to God, Christ, the kingdom that is set up, and most importantly...life. Our reward is life aionios. So whatever duration or quality life is as "everlasting" so is anything else that is "everlasting" (aionios).
Mat 18:8 - Wherefore if thy hand or thy foot offend thee, cut them off, and cast them from thee: it is better for thee to enter into life halt or maimed, rather than having two hands or two feet to be cast into everlasting (aionios) fire.
Mat 25:41 - Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting (aionios) fire, prepared for the devil and his angels:
Mat 25:46 - And these shall go away into everlasting (aionios) punishment: but the righteous into life eternal (aionios).
Luk 16:9 - And I say unto you, Make to yourselves friends of the mammon of unrighteousness; that, when ye fail, they may receive you into everlasting (aionios) habitations.
2Th 1:9 - Who shall be punished with everlasting (aionios) destruction from the presence of the Lord, and from the glory of his power;
While three of these are explicitly within the last days, and therefore apocalyptic , context, that does not necessarily alter or diminish their meaning. Note particularly Matt. 25:46. The word aionios is translated as both everlasting and eternal. Eternal throughout the NT is translated from this word. So the nature of God and the nature of both our reward for being faithful and our reward for being unfaithful are wrapped up in this word. If Gehenna is therefore limited so is Heaven and even God.
Note also Luke 16:9. Those who make themselves friends of the things of men and unrighteousness, when they fail, those friends will receive them into eternal habitations [literally tents, tabernacles, or dwelling places].
I'm convinced that, despite whatever nuances there may be in the words fire and destruction, punishment for disobedience once we leave this temporal life is as eternal in quality as Heaven. Not ongoing in the sense of time that does not stop, but more than that, superior to time or not limited by time.
One final note from logic. I believe that God's justice requires equal and opposite rewards for obedience and disobedience. It would be unjust to mete out unlimited reward for right doing and only limited reward for wrong doing. The stakes are eternal either way and it is only in this temporal existence that we choose that eternal destination.
In Truth and Love.