Question

What is the function of metaphor in this line from the passage?

Answer 1

I believe the correct answer is It compares an experience to a method of torture.
The author is trying to compare slavery to hell - he isn't comparing it to the actual physical gate, but rather what lies beyond the gate. He thinks slavery, and what slaves had to go through is akin to hell, or at least our depiction of it.

Answer 2

The second sentence

It compares an experience to a physical gate