It is discouraging to see that there are so many who completely misunderstand this topic.
While we do not have all the answers, Scripture is quite clear that the 2nd death is forever, irreversible, eternal, without end, etc.
Some have reasoned that eternal punishment does not fit the crime. Based on what scale of justice? Our reasoning versus God's infinite knowledge? He knows every thought we've had and ever will have; He knows how every action and reaction we make(including the motives behind them) will have affected others and the actions/decisions they make, and so on.
While by our definition torture is a form of punishment, the punishment God prescribes to the various unsaved individuals will be perfect and unique in magnitude to each person. You can't have imperfect punishment from a perfect God. If God sent individuals to be tormented in Gehenna/Hell and then afterwards allows them into His kingdom after repenting, then that would amount to God torturing them in order to get His way with them. Their repentance would be based on them not wanting to remain in the torment of Hell rather than the sacrifice He made on the cross.
He is also a God of love. He wants everyone to repent and accept Jesus Christ as Savior, change their minds about sin(repent), and be able to dwell with Him forever. However, with that being said, it should also be added that He loves us so much that He would give us the free will to choose and that one's final state in Hell will be determined by weighing how they lived their lives versus how much knowledge of the Gospel they have received. Every individual, both saved and unsaved, will be held accountable: for the saved, it will be a judgment of their works; later on for the unsaved, it will be a judgment of all their sins since they did not accept Christ.
As I said before, we do not have all of the answers in regards to how God is going to, for example, judge those who never heard the Gospel. The thing to keep in mind is that God knows everything that was, is, is to come, and what "could've been" both with and without His intervention for each case.