"Spend 2 Gas for each Gas not spent. Lose 1 Survivor."
Does that mean, if I don't spend the 2 gas to drive away, I need to spend 4 gas? Does it mean that if I have 6 gas tokens in my hand, I need to give away 4 and keep 2? Or Does it mean something else entirely?
You need to give up 2 gas tokens. For each one you can't or don't pay you lose 1 survivor. If you pay 2 gas you lose no survivors, if you pay 1 gas you lose 1 survivor and if you pay no gas you lose 2 survivors.