In what sense might Gilgamesh's quest be understood as a religious or "spiritual" quest? How does Gilgamesh's story correspond with the Five Stages of the Soul (Call, Search, Struggle, Breakthrough, Return)? The stage of the Struggle often involves people trying to discourage the Hero from continuing on the quest. What obstacles does Gilgamesh encounter on his quest to find immortality? In losing the plant, Gilgamesh loses immortality. What wisdom does he possibly gain in its place?

First, we have to understand that the concept of "spiritual" is different from culture to culture and especially, time after time. Anyway, the "spiritual quest" in Gilgamesh can be sum up into three stages:

The identity of the character - Who am I?
The purpose of his life - What should I do?
The meaning of life or happiness - What fulfill me?

Gilgamesh's quest involve answer these questions, and find a way to explain himself. That's why we can describe his journey as a spiritual quest. The journey he undertakes, if separated into the Stages of Soul, then we have:

Call - Gilgamesh fells that must seek the plant of immortality
Search - He and Enkidu undertake an adventure to find the plant
Struggle - several monsters and obstacles, but also, Enkidu's murder by the envy gods. Gilgamesh must continue the quest without his friend.
Breakthrough is painful, and Gilgamesh loses the plant.
He returns home with the conviction of his mortality.

Maybe the better answer to the message of this tale is that life is precious, and you must live plenty. You see, Gilgamesh was a futile and lazy king, but also vicious and violent. He was wasting his life, so he finally realizes it's importance, but also learn that loneliness is painful. He never care about anyone but him. When Enkidu dies, he understands the value of a friend.

Explanation:

The Epic of Gilgamesh is a classic tale of return carrying a message that was obtained through terrible grief and struggle. But also, a personal point of view of the human soul.