A car running out of gas on a circular track

Imagine an additional similar car B with a very large tank which has more than enough gas to complete a lap. Let the car B start from an arbitrary gas station on the circular track, buy all the gas from the gas stations it encounters, and complete a lap. Let $G _1$ ​ be a gas stations where the level of gas in the car B is the lowest when B arrives at $G_1$. If the car $A$ with an empty tank starts from $G_1$ ​ and travels in the same direction as the car B, then it will be able to complete a lap by buying gas from the gas stations it encounters along the way. Indeed, if A runs out of gas between the gas stations $G_i$  ​and $G_{i+1}$ ​ for some $1\leq i \leq n-1 $, then the amount of gas required to travel from $G_1$ ​ to $G_{i+1}$ ​ is more than the total amount of gas available in stations $G_1, \cdots, G_{i+1}$. This implies that the level of gas in the car B at station $G_i$ ​ was strictly lower than the level of gas when it arrived at station $G_1$. This is a contradiction and the proof is complete.