You are asked to calculate gas mileage for a car, and your answer comes out with units of gal/mi. this means that

If you were to calculate the gas mileage of your car and the answer turns out to be in units of gallons per mile, then that simply means your car uses a gallon of gas per mile. So for every one mile of traveling on the road that you do, you would make use of an entire gallon. This is the standard unit of measuring gas (also known as MPG). If  you have a lower amount, then that means your car uses less gallons per mile which means that it is more fuel efficient. The opposite can be said for when it is higher.