The Mercedes G63 AMG 6X6 indeed had two fuel tanks, meaning that when Gao Yang finally seized a vehicle and fled, his decision to abandon the car due to lack of fuel was totally unnecessary. All he needed to do was flip a tiny switch, and he would have avoided the dangerous predicament of nearly being killed.
One could only say that fate works in mysterious ways. If Gao Yang had driven off in the car at that time, he wouldn't have come across that Clemens-made shotgun that thrilled Morgan so much, and without that shotgun, Morgan certainly wouldn't have insisted on giving him the car.