In the great gatsby, what does the green light on the harbor symbolize?

It symbolizes him looking across looking at her dock.

The green light symbolizes ambition, desire, and longing. This is the longing to belong to a different class, a different life, a different identity. The book ends with an image of boats beating against the current, desperately striving to reach something (like the green light across the harbor) even as the current carries us back and back to where we came from.