I reviewed the script to refresh my memory. Here's what needs to be done:

1) Visit the prophet and give him the Dragon Sword (or something similar), which is obtained in Utopia.
2) Guide the main hero to the gate and get as close to it as possible.
3) The hero MUST be wearing the Teleportation Ring (also known as the Ring of Exile).

Keep in mind that you can only pass through the gate once. If you try to "cheat" and use a portal to the city after passing through, only the console will help.