The Gulf is a large impassible expanse that is established between the good side of Heaven and the bad side, and it is a creation that exists only in this present age, the second heaven and earth age, and will cease too exist after this earth age. On the good side of Heaven, those there are in the presence of God and have no wants or needs. Those people have lived life the way they were supposed to live it and followed God’s ways and accepted Jesus Christ as their Lord and Savior.
On the bad side of the Gulf, known as Hades in the Greek and Sheol in the Hebrew and translated as Hell, all those who have not overcome the ways of the world, before their deaths in the flesh, are put. This is a holding place in which those people are away from the presence of God. These people burn with shame and thirst for the Water of Life that would have come to them with their sincere acceptance of Jesus Christ.
There is no way for someone to simply pass from one side of this Gulf to the other, however, after Jesus was crucified and before He was resurrected, He went to the gates of Hell and preached to those souls there to allow them to accept salvation (Jonah 1:1-3:10, 1Pet 3:18-22, 4:1-6). Although this expanse is presently not able to be traversed, each side is not so far away that one cannot see to the other side and know what goes on there. It is also likely that many of those currently held in Hell will receive a chance to correct their ways in the Millennium, especially if they had never gotten a chance to hear the Gospel while in the flesh.
Those on the side called Hell see what they are missing in that they are not themselves in the presence of God, and because they can see this, they grieve that they had not made the right choices while in the flesh. The people on the heavenly side can see those on the hellish side and see their suffering of not having God with them. Knowing the love of God, but feeling that God is totally apart from you is the worst feeling in the world, and this is the punishment that all on the side of Hell will feel. For those who are on the bad side, the Gulf is their separation from God and all goodness and love; and for those on the good side, this Gulf is a separation from any suffering or sadness. (Luke 16:19-31)