Gehenna was a place where children were sacrificed to false gods. That is why Jesus used it to describe hell.
Gehenna (translated as hell) was Jesus’ standard metaphor for hell:
Matthew 10:28 And do not fear those who kill the body but cannot kill the soul; rather fear him who can destroy both soul and body in hell. (cf. 5:22, 29-30; Mk 9:43, 45, 47)
Matthew 18:9 And if your eye causes you to sin, pluck it out and throw it away; it is better for you to enter life with one eye than with two eyes to be thrown into the hell of fire.
Matthew 23:15 Woe to you, scribes and Pharisees, hypocrites! for you traverse sea and land to make a single proselyte, and when he becomes a proselyte, you make him twice as much a child of hell as yourselves.
Matthew 23:33 You serpents, you brood of vipers, how are you to escape being sentenced to hell?
Luke 12:5 But I will warn you whom to fear: fear him who, after he has killed, has power to cast into hell; yes, I tell you, fear him!
But it was not a place of constantly burning garbage, as is often thought. Rather, it was a place where children were sacrificed to false gods. That is why Jesus used it to describe hell. Some strains of rabbinical Judaism also did so.
Gehenna is a reference to the Valley of Hinnom, which is directly south of Mt. Zion, near the Old City of Jerusalem. The area is described in Joshua 15:8 and 18:16. When I was in Israel in October 2014, I walked down a good portion of the Kidron Valley (after walking around the entire Old City), next to the Temple Mount and ancient City of David, down to the Pool of Siloam.
This is perpendicular to the Valley of Hinnom. But I didn’t go further into that area, due to the significant unrest in the Muslim portions of Jerusalem when we were there. It would have been strange indeed to walk the valley that in Jesus’ mind was akin to hell itself, due to the ritual childkilling that so often took place there.
In the richly metaphorical Jewish mind, high places were often seen as more holy (Mt. Zion and Mt. Moriah, where the temple was, Mt. Sinai, and on and on). Thus it stands to reason that low places (i.e., valleys) would come to also symbolize Sheol and later, hell. Jesus, therefore, chose this horrible place as His description of hell
The most unholy thing imaginable in the Jewish mind (including the mind of Jesus, we might surmise), was the sacrificing of one’s own children to be burned to death in a furnace in order to please some false god.
We know from biblical history that child sacrifice to Ba’al or Molech (which is what abortion is at bottom: a person’s own baby sacrificed to the idolatrous gods of greed, hedonism, or sexual “freedom”) took place there:
2 Kings 23:10 (RSV) And he [King Josiah] defiled To’pheth, which is in the valley of the sons of Hinnom, that no one might burn his son or his daughter as an offering to Molech.
2 Chronicles 28:3 and he [King Ahaz] burned incense in the valley of the son of Hinnom, and burned his sons as an offering, according to the abominable practices of the nations whom the LORD drove out before the people of Israel.
2 Chronicles 33:6 And he [King Manasseh] burned his sons as an offering in the valley of the son of Hinnom, . . .
Jeremiah 7:30-32 "For the sons of Judah have done evil in my sight, says the LORD; they have set their abominations in the house which is called by my name, to defile it.  And they have built the high place of Topheth, which is in the valley of the son of Hinnom, to burn their sons and their daughters in the fire; which I did not command, nor did it come into my mind.  Therefore, behold, the days are coming, says the LORD, when it will no more be called Topheth, or the valley of the son of Hinnom, but the valley of Slaughter: . . .
Jeremiah 19:2-7 and go out to the valley of the son of Hinnom at the entry of the Potsherd Gate, and proclaim there the words that I tell you.  You shall say, `Hear the word of the LORD, O kings of Judah and inhabitants of Jerusalem. Thus says the LORD of hosts, the God of Israel, Behold, I am bringing such evil upon this place that the ears of every one who hears of it will tingle.  Because the people have forsaken me, and have profaned this place by burning incense in it to other gods whom neither they nor their fathers nor the kings of Judah have known; and because they have filled this place with the blood of innocents,  and have built the high places of Ba’al to burn their sons in the fire as burnt offerings to Ba’al, which I did not command or decree, nor did it come into my mind;  therefore, behold, days are coming, says the LORD, when this place shall no more be called Topheth, or the valley of the son of Hinnom, but the valley of Slaughter.  And in this place I will make void the plans of Judah and Jerusalem, and will cause their people to fall by the sword before their enemies, and by the hand of those who seek their life. I will give their dead bodies for food to the birds of the air and to the beasts of the earth.
Jeremiah 32:35 They built the high places of Ba’al in the valley of the son of Hinnom, to offer up their sons and daughters to Molech, though I did not command them, nor did it enter into my mind, that they should do this abomination, to cause Judah to sin.