To understand what Jesus meant by Gehenna, I suggest we need to consider its meaning in the context of scripture, rather than Jewish myths and fables, that Paul warned against in Titus 1:14.
The comments of Jesus about Gehenna were based upon scripture, and especially the prophecies of Jeremiah, not the traditions of men. Jesus said of the scribes and Pharisees, they "transgress the commandment of God" by their tradition. [Matthew 15:3]
Zechariah said that all the land round about Jerusalem would become a plain, and Jerusalem would be lifted up.
Zechariah 14:10-11All the land shall be turned as a plain from Geba to Rimmon south of Jerusalem: and it shall be lifted up, and inhabited in her place, from Benjamin's gate unto the place of the first gate, unto the corner gate, and from the tower of Hananeel unto the king's winepresses.
And men shall dwell in it, and there shall be no more utter destruction; but Jerusalem shall be safely inhabited.
The idea that "Jerusalem shall be safely inhabited," points to Jerusalem as the dwelling place of those who have obtained salvation. The prophecy applies to the heavenly Jerusalem. The earthly city offered the Jews no safety in 70 AD.
The places mentioned above, Benjamin's gate, and the corner gate, are east-west extremities of the city, and the tower of Hananeel to the king's winepresses were the northern and southern limits of the city; the limits specified seem to exclude the valley of Hinnom. That would be in the region that becomes a plain.
Zechariah's prophecy compares with the prophecy of Isaiah, who also said Jerusalem and Zion would be exulted above the hills. [Isaiah 2:1-3] In the New Testament these prophecies are fulfilled as Jerusalem is in heaven, and the earthly Jerusalem is identified with Hagar the Egyptian bondwoman who was cast out of Abraham's house. [Galatians 4:24-31]
In Zechariah, the nations coming against Jerusalem corresponds to Revelation 20:8-9, where deceived people from all nations come against "the camp of the saints" and "the beloved city," which is the new Jerusalem, the church.
As for those nations who come against Jerusalem, In Zechariah 14, they were threatened with a plague.
Zechariah 14:12And this shall be the plague wherewith the LORD will smite all the people that have fought against Jerusalem; Their flesh shall consume away while they stand upon their feet, and their eyes shall consume away in their holes, and their tongue shall consume away in their mouth.
"Their tongue shall consume away in their mouth," IMO is metaphorical. It applied to the Jews who were unable to reply when Jesus asked them a question, which they were unable to answer.
Matthew 22:46And no man was able to answer him a word, neither durst any man from that day forth ask him any more questions.
Their eyes consuming away in their holes is also metaphorical; Jesus called the scribes and Pharisees "blind guides." [Matthew 23:16, 24]
He said they "shut up the kingdom of heaven against men: for ye neither go in yourselves, neither suffer ye them that are entering to go in." [Matthew 23:13]
Thus, their eyes consuming away in their holes refers to spiritual blindness.
This helps to show why Jesus said they were in peril of the threat of Gehenna in Matthew 23:33; Jerusalem represents the kingdom of heaven, and they not only did not get into it themselves, but hindered others from doing so.
Zechariah said that the nations which came against Jerusalem would fight each other.
Zechariah 14:13And it shall come to pass in that day, that a great tumult from the LORD shall be among them; and they shall lay hold every one on the hand of his neighbour, and his hand shall rise up against the hand of his neighbour.
This "great tumult" is evident amongst Christians, who seek to interpret prophecy, yet cling to their traditions. Dispensationalists at war with Preterists; partial and hyper-Preterists at war; classical Dispensationalists, progressive Dispensationalists, ultra-Dispensationalists, mid-Acts Dispensationalists, etc., all at war. And tens of thousands of denominations. Clearly, this prophecy is being fulfilled.
DougGehenna and the land promise