How does the gas station problem use the greedy approach?

If the total gas across all stations is at least the total cost, a solution exists. To find the starting station, iterate through all stations tracking the current fuel balance. When the balance drops below zero, the start cannot be at or before the current station, so reset the balance and try the next station as the new candidate start. After one pass, the last candidate is the answer. This works because if a segment of the route causes a deficit, no station within that segment can be the start.