Once a mortal, he traveled the world in search of something that would bring him the fulfillment that all mortals seek. Not finding anything that would bring him pleasure, he started meditating in the hollow of a tree in the middle of a vast forest and started to contemplate the world around him.
So deep was his contemplation that he could not snap out of it. After nearly a century had passed, he woke up in a new form, an enlightened form. He then realized that he was no longer stuck in the shell of a human body, but was in a new, more powerful, enlightened form.
Gandor generally remains visible, but once in a while he goes into hiding for a few days to return to his contemplation to see if there is anything else to learn.