The city of Jerusalem is one of the most prominent of all places mentioned in scripture, and in the New Testament, Jerusalem is the name of the church, and its location is heaven, not upon the earth. The various locations and landforms associated with the promised land are the setting for the revelations of God in scripture. These revelations include the gospel, and the promise and hope of the future resurrection.
Jesus identified himself with God’s temple. In the earthly Jerusalem, the temple was in the city, but in the New Jerusalem in Revelation 21, the city is in the temple. John wrote that he saw no temple in the heavenly Jerusalem, but said “the Lamb was the temple of it.” In prophecy, the holy city and the temple are intricately associated. W. D. Davies wrote: 
...the pertinent texts move without warning from the temple to Jerusalem and vice versa, so that these two entities, in their earthly and heavenly forms, are in constant association. … And, since the texts dealing with the Temple always implicitly, and usually explicitly, implicate the city, just as Jerusalem became the quintessence of the land, so also the Temple became the quintessence of Jerusalem.
When Jesus was raised up to heaven, as described in Acts 1:9, the holy city was raised up together with him, fulfilling the prophecy of Isaiah which said the mountain of the Lord’s house would be exulted above the hills, to the tops of the mountains. [Isaiah 1:1-3]
Gehenna is the valley of Hinnom, a short valley in the promised land, to the south and west of Jerusalem. In Hebrews 4, the promised land is a type and shadow of the “rest” of the saints, and the hope of salvation, and also of spiritual blessings promised to the saints.
The valley of Hinnom is the subject of lengthy prophecies in the book of Jeremiah. From the frequent mention of it in scripture, it seems to be among the most prominent valleys in the Promised Land. In the sayings of Jesus, Gehenna represents a judgment, as do other valleys mentioned in prophecy. The valley of Jehoshaphat, in Joel 3, also called “the valley of decision,” is one. Tradition associates this with the Kidron Valley that lies between Jerusalem and the mount of Olives. In the New Testament, the valley of Meddigo, or Armageddon, has a similar role, as both sites are places where the nations are gathered together, and God pleads with them on behalf of his people.
The valley of Gehenna is near Jerusalem, which represents the camp of the saints. To be cast into it means being excluded from the holy city, because one is deemed unfit for the kingdom of God. The walls of Jerusalem represent salvation, and Gehenna lies outside the walls of the city.
Joel 3:18 says that all the ravines of Judah will flow with water. This would include the valley of Hinnom, or Gehenna. In the same prophecy, the hills will flow with milk, and the mountains will drop down new wine. Here, the milk and new wine represent the truths of the gospel. The apostle Peter said, “As newborn babes, desire the sincere milk of the word, that ye may grow thereby.” [1 Peter 2:2]
New wine suggests new understanding from the scriptures. Jesus said, “And no man putteth new wine into old bottles: else the new wine doth burst the bottles, and the wine is spilled, and the bottles will be marred: but new wine must be put into new bottles.” [Mark 2:22] He was referring to his teachings, which were new revelations. They included things that had never been previously understood. Jesus said to his disciples, “But blessed are your eyes, for they see: and your ears, for they hear. For verily I say unto you, That many prophets and righteous men have desired to see those things which ye see, and have not seen them; and to hear those things which ye hear, and have not heard them.” [Matthew 13:16-17]
Joel's prophecy about the valleys of Judah suggests that even those cast into Gehenna will obtain mercy, and will come to repentance, and worship God.
Jeremiah 31:38-40 describes an extension of the boundaries of Jerusalem. According to this prophecy, the valley of Gehenna will be included within the extended boundaries of the holy city. The region includes the valley of Hinnom, called the “valley of the dead bodies.” Jeremiah says it will become “holy unto the Lord.” This prophecy also implies there is hope for those cast into Gehenna.
1. W. D. Davies. The Gospel and the Land: Early Christianity and Jewish Territorial Doctrine. U. of California Press 1974. pp. 150-152.